I was looking at the flaw Infamous Master, and it occurred to me that even with a terrible reputation, there is no suggestion that he had been kicked out of his House.
While I suppose you could be an exemplary mage in all ways, except being friendless and having truly annoyed your Primus, surely there are other examples of Magi being declared Orbus from their House?
Each House probably has their own criteria as to what it takes to be kicked out of the House.
Criamon who abandon their quest for enigmatic wisdom get declared Orbus and given a year to find another house - see HoH:MC on Gorgiasts.
I imagine Tremere don't get declared Orbus - it is more likely one of the three Arcane Connections fixed to you will be taken out of storage and you will either somehow find yourself behaving more appropriately (Mentem effect) or mysteriously disappear like an old oak table.
You need to do something that is offensive enough to the House to kick you out, without declaring a Wizard's War... that's kind of a tough call. It is like "You did something really bad, but not just bad enough to try to kill you. And it does not break the code." Or maybe because the mage is too powerful to be WW by anybody of the House.
The easiest case I could see is an agreement on both side: the mage express the need to change House because he found a stronger call in another House (mysticism, need to go on Crusade...). Then it is a kind of gentlemen agreement. No bad blood. Especially if the leaving member does not know Inner House secret.
Then some suggestions:
Refusing to honour Primus request or follow House recommandation during a Tribunal could be a ground for being declared Orbus without being WW: it is a moral offense, but probably not worth a death sentence.
Making a fool of himself to the point of embarrassing the House (a Bonisagus claiming somebody else work as his own and being discovered ?). Making a serious mistake, fudging proof, breach of confidence for a Guernicus.
Probably House Tytalus and Ex Miscellanea would never cast out somebody from their rank. House Tytalus because somebody would find challenging to bring the mage back inline (if ever a line exist in this House), House Ex Miscellanea because nobody remembered that the mage was part of the House.
A Bjornaer might be declared Orbus because he lost his heartbeast, though pity would be a more probable result. Sharing House secrets with non-Bjornaer would be too severe a crime and would probably result in a Wizard's War instead.
A Bonisagus who does no research, or refuses to share the result of his research, might be declared Orbus.
As mentioned, a Criamon who becomes a gorgiast would be declared Orbus.
A Flambeau could be declared Orbus because he displayed cowardise in the face of the enemy.
A Guernicus who abuses his status as a Quaesitor repeatedly, though not severely enough to be Marched, might be declared Orbus.
A Tremere who repeatedlt refuses to obey his superiors' order, or shows disrespect to them, might be declared Orbus. Provided that he doesn't know enough to be a threat to his House, in which case he would probable be targeted by Wizard's War.
It is unlikely that a Jerbiton, Mercere or Tytalus would ever be declared Orbus. As Houses Merinita and Verditius, one is unlikely to be declared Orbus without also being subject to deadly attempts on their life, either direct (Wizard's War) or indirect (through the agression of faeries).
YSMV
I agree with your other points, but I could see these Houses declaring someone Orbus.
Well, I can see a Jerbiton, Mercere or Tytalus Orbus.
Tytalus most easily - any magus that would be seen as weak (not so much magically as a matter of character) could be expelled, and there could be elaborate games afoot to engineer such fates for your rivals.
Jerbiton - I think any magus that's seen as extremely vulgar or crude may find himself expelled from this House. Keeping him in the House would be an insult to the House as a civilized society. Now, he'd have to make a nuisance of himself to the House for the House to bother with it. Or else have enemies in the House that will bring the matter up with the Primus. But that doesn't take much. It could be as simple as a Jerbiton that wins a Flambeau Tourney, and having his enemies in-House use this to attempt to get the Primus to expel him.
Mercere - well, if you bring ill-repute to the House that may harm its business (vis banking & communications), I could see the house expelling you. You would basically get a year to "run and hide", as presumably not even Ex Misc would accept a mundane redcap. But then again, no magus worthy of his name would go to the effort of Marching you, too. So you'd be a man with an open death sentence on him, but probably not actively pursued, which seems fitting as punishment.
As for Merinitia and Verditius, I tend to agree. No real reason to expel someone from these Houses.
Verditius:
going too far in a vendetta (killing/injuring the opponent)
regularly selling items below house price - even after warnings and sanctions
Merinita:
Creating and spreading a negative story about someone senior
Altering active faerie stories to the detriment of the house
Encouraging fae to intervene in the Order.
Tytalus: the magus who takes up the challenge of being expelled but not WW
For Tremere, it's explicitly declared (in the Guernicus section of HoH:TL, p.48, Proxy Voting box) that the way Tremere do the vote sigil thing under the Code is that it's counted as a free proxy. Which means under the Code, you can in fact demand your sigil back at any time -- but if you do without winning it, the House declares you an Orbus.
A Redcap who "blatantly and continually flouts his responsibilities", even after a warning from the Primus, also risks getting declared an Orbus (HoH:TL, p.92). "[A]s no other House
is likely to adopt the Redcap . . . if Mercere does not accept him back within the year it is effectively a death sentence, since he will be formally cast out of the Order."
Almost, but not quite. Criamon magi assume that whoever want to abandon the quest for the Enigma wants to abandon the House (which is all about the quest for the Enigma, after all), but such a magus does not get immediately get declared Orbus: "the House avoids renouncing magi until they have arranged adoption into another House."
GotF, p.17, gives an example of a Bonisagus maga who was declared orbus (sic - orba, I'd say) in the mid-11th century. Aschlaranda was part of a group of Bonisagus magi who had been considered Diedne sympathyzers. Absent from the Tribunal for several decades, she reapperared at Durenmar in 1048. "She was accepted back into House Bonisagus, but her actions and opinions were contrary to the prevailing spirit of reconciliation... After a major dispute, she was (with reluctance) declared orbus." Aschlaranda eventually joined ex Miscellanea, and founded the Dankmar covenant which Schadrit, who had been Aschlaranda's apprentice at the time of her casting out, still rules with an iron fist in 1220.
I would add that, ultimately, the power of casting someone out of their House rests, by Code and tradition, solidly within the hands of the Primus; it is, in fact, ultimately the strongest weapon in the hands of Primi from disorganized Houses like Ex Miscellanea and/or "democratic" ones like Jerbiton. This means that a magus with the strong disfavour of the Primus could be cast out of the House "just because"; whereas a magus with the support of his Primus is unlikely to be declared an orbus ... unless he is so hated by the rest of the House that the Primus is really under a lot of pressure. Also, note that a magus who is declared orbus remains a member of the Order for (at least) a year, and so can still declare lawful Wizard War on the Primus that cast him out; slaying the Primus does not reinstate him, but the new Primus might ....
A Bonisagus who refuses to share their research is violating their Oath. That's grounds for a March, in theory. A Bonisagus can probably dodge this with a convincing declaration that their research is not finished, or not ready for publication. Unless there's convincing evidence the mage's research is being put to use, it's difficult to prove otherwise.
It almost sounds like it is more common for a Magus to renounce their House and become Orbus, as it is for a House to renounce a member.
Silly hypothetical: a Flambeau mage, member of the school of Flambeau, gets the Flaw Incompatible Arts - CrIg from a bad Twilight incident. Quits the House in shame.
In theory a Bonisagus could keep the oath by allowing other magi to examine his untranslated lab notes. Of course this means that he could potentially be hounded by people wanting to spend multiple seasons translating his notes...
As a policy matter, Bjornaer, Criamon and Tytalus don't declare orbi, ever.
Bjornaer doesn't let you leave alive. If you're declared a non-Bjornaer, then you'll be Wizard's Warred by the entire House.
Criamon are absolutely against the ruling that "a magus must have a House," due to their principle of nonviolence, so they will not declare an orbus. They will, in fact, accept someone who is orbus from Ex Miscellanea or otherwise unable to get a new House - this results in House Guest lineages, most of which don't seek the Enigma.
(What happens if a rogue Redcap applies for Criamon membership? A story seed.)
Tytalus doesn't cast you out because you only cease to be a Tytalus if you cease to follow their philosophy, and only you can determine that. (Also, right now Tytalus doesn't have anyone who can make the call that you're orbus.)
Jerbiton, Merinita and Ex Misc don't throw you out because the Houses in question aren't active power blocs; they're loose groupings of people with shared interests. The Primus theoretically might do so (and in the case of Ex Misc, this would be a major problem), but frankly the Primus probably doesn't know you exist, and a Merinita Orbus would probably be able to just join Ex Misc and continue as a Merinita so long as they didn't cheese off their own subtradition. That said, if a Merinita pisses off their own in-group, there are likely to be consequences.
Mercere will fire Redcaps who don't do their job, or magi Mercere who don't obey the House's traditions (have children before you take the Potion, don't teach non-descendants, don't vote in Tribunal...).
Verditius will throw you out for disobeying pricing rules. Leaving Verditius is permitted, but you are doubleplusexpected not to share House secrets on pain of Wizard's War.
This is about correct.
What would it take to found a new house instead of becoming orbus?
I think you'd have to win a vote at Grand Tribunal to found a new House. And to do that, you'll probably have to convince a Tribunal to make your proposal one of the three issues they are choosing to raise at Grand Tribunal. The alternative is convincing the Bonisagus Praeco that your proposal deserves to be on the agenda. And then there's the daunting challenge of winning the vote, when few Houses or Tribunals have anything to gain by the addtion of a new House.
But it's a great story!
You can always join ExMisc and start a sub-tradition. Aside from representation at the Grand Tribunal, you're practically a House. It's not as if the ExMisc Primus is going to bother you from his lair in Stonehenge.
Creating a new House is the stuff of legend - if you consider that it only happened once in the whole Order history, when Ordo Miscellanea became House Ex Miscellanea.
To create a new House, a mage should have a large number of future new members, an agenda that would be different than any of the existing House and have the support of at least half of the existing Houses. Or some very strong levy.
You have to see that the consequence of creating a new House are very deep: it changes the balance of power, there will be 13 houses, changing politics at Grand Tribunal. Probably that a large number of new members House will be coming from other House, which will deprieve them from power, expertise and knowledge, unless the new house would contain at least 60-80 members coming from another tradition, and powerful enough to be consider worth becoming member of the Order before learning the secret of Parma and hermetic theory.
One occurence I could see this happening would be the creation of an House dedicated to all three monotheist religions, with relevant abilities (Holy Magic, muslim sahir), with interactions and links with their relevant Church. Such future House, even with a relatively small number of members (let's say 30ish), would have enough levy and external protection to make the whole Order pauses before going for complete obliteration. Of course, there will be the whole aspect of being mage without oath of fealthy outside the Order, but that would be part of the many details to iron out for the creation of such House.