Yummy. For me I would say Jerbiton, Bonisagus and Flambeau, because they are the paradigmatic magus that interacts with society, the psycho that does his stuff and the lab rat. oversimplifying, but it is how they are perceived in broad strokes. None of them needs toe OoH (or other magi, for that matter) to exist at all to ply their role.
Been there, done that. The OoH works quite well with smaller traditions. A large Exmisc-like Order. We did it by removing hermetic Magic from the setting, though, so somewhat more extreme (and more complicated rules wise) than just dropping the houses. The houses as written do not make much sense to us. They are not political affiliations, they cannot affect the life of their magi much and they do not really encompass pan-european magi since tribunals beat them hands down in their influence on the local magi. CDalling them "traditions" more than houses would be better IMO. It is not a big issue in any case
OTOH I find houses extremely useful when starting to play because they are basically predefined in their roles. And there are only 12 templates to choose from, so it offers diversity but not too much choice when selecting a magus to play. That is very useful for new players: The lab rat, the policeman, the messenger, the soldier, the faeries guy, the animal guy, the smith, the weirdo, the the killer, the jerk, the social guy, and the outcast. All oversimplifications and the setting offers much more than that, but as a starting list of "things to do with your magus", they are clear and easy to chose from.
been there, done that as well. Not because it is difficult, but because it does not make a lot of sense and we do not like the current explanations regarding faeries and vitality. We have 2 realms, the Physical (Legend) and the Soul (Divine) realms. We did that because we did not agree about the way faeries are described in Ars, and because the line between fae and magic is extremely blurry in the current setting. Current RAW describes a lot of faerie beings that could be magical and magical beings that could be faerie with the same ease, so it is not a big problem dumping them all together. The fae-magic realm difference is a merinita internal debate IMS. Having done that, we went heretic and detemined that God is the ultimate boss of demons as well, so we dumped both demons and angels in the same realm from a hermetic magic point of view: hermetics are affected equally by both realm auras (and in our saga they cannot tell the difference between the 2 either), so their theory says that they are the same. Demons and angels would not agree though, but neither would a librarian dragon and a pixie while being told that they are both faeries ad basically the same thing.
Not necessarily. We find that mechanics in 5th edition are over the top in cumbersomeness, and some of the setting definitions are weird. it is the best edition, yes, but not all fits what our troupe finds palatable. I assume most other troupes have similar issues. We have subsystems and extra rules for everything. The setting is rich, but the way some issues have been solved are not ideal in my troupe's opinion, so they get changed. A lot of people does that. And we are all still playing in Mythic Europe, the best setting ever.