I was wondering whether it is possible to hit a target you are not explicitly aware of (e.g. a invisible spirit or demon) by casting a spell on one of the 'area-targets' I mentioned in the headline.
On the one hand, I think it should. If I cast a PeCo spell at T:structure every humanoid within the building should become victim of the spell, whether I see them or not, otherwise the target doesn't make much sense: Casting a spell on the building itself reqires T:Individual with increased size, and if I only want to affect the people I can see anyway, T:Group would be sufficient. All this is even more true for T:Boundary and also makes sense for T:room.
But then, casting spells at physical beings is not considered very difficult, here it seems to fit. On the contrary, hitting a demon with magic is mostly limited by te fact that you can't see (or otherwise perceive) a demon that is hiding (him being a creature of lie and deception). Unless I plan to use really mighty demons (Might 50+) this is the main danger of a demonic plot. Now, much of this potential is destroyed if a middle aged magus with a decent PeVi score could cast an unstressful spell every hour or so, to make sure that no minor demon (let's say up to might 15) is in the house, hell I could even imagine a Verditius who crafts items which do so on a round by round base.
So, what to make of this dilemma?
No mechanical problems with your structure/boundary/room indirect spells. perfectly feasible IMO. Such regular and constant spellscasting in such circumstances is likely to warp the room and everything in it, though.
You'll need to keep your demons outside I guess. News may travel quickly throughout the heirarchy that these magi nuke rooms with demon's eternal oblivions on a regular basis.
Keeping out minor supernatural creatures is usually done by the Aegis...
So no big deal for the convent. While travelling I could use wards.
As for the indirect spells, I would agree with you.
If you need a demon to enter the Aegis there are several ways to archieve this:
Posses a person and enter the Aegis with him. As long as the demon does not use any other powers he is usually not expelled by the Aegis, though his possible actions are limited.
Send in a minion/diabolist to do the mischief. Since they have no might, they are not hindered.
Force/Persuade the target to leave the Aegis by initiating some events outside the convent he wants to see..
In one of our sagas we had a demon impersonate a grog and PARTICIPATE in the Aegis ritual. It was great fun to have a fairly powerful demon unaffected by the aegis during the next year. It was played by the beta SG and until we unravelled the reason of our misshaps he did quite some damage, the bastard. Loads of fun there
Since then, making sure that the people taking part in the ritual do not have magic resistance (except for the magi) has been quite a common feature in our sagas.
We just came across this in our saga - there was an Aegis up, but someone discovered a demon inside the aegis. After the demon was destroyed the aegis vanished. It turns out that it was a powerful deceiving demon and it was making us think that there was an aegis. We don't know how long it was there, causing minor mischief and fooling us.
There is currently a massive effort to catalog everything in the vaults to make sure that nothing has been added, removed or tampered with. Every document in the place is now suspect and some of the best "auditors" from house Mercere are on their way. Who knows what little scams the demon has been covering up? Some items in the place disappeared completely when the demon was destroyed. Even the most loyal grog would be tempted to take something if no-one notices....
