It seems that demons functionally have unlimited Magic Resistance against Hermetic Magic, and any other magic that relies on being sensed in order to effect. When they turn into their spiritual form, there's no real Hermetic method to detect them (Limit of Infernal) and as far as I can tell, no way Holy Magic (via a Hermetic magus) can detect them without a Ritual.
What is the point of Sense Holiness and Unholiness if it has to Penetrate Magic Resistance? Regarding demons, Second Sight is an Ease Factor of (9 + magnitude of Demon Might), ditto there...
There's continual reference in RoP:I to "Divine Powers" that can reveal a demon in spiritual form... but I can't seem to find any in RoP:D that actually can do that. There's the saintly power of Expel Demons (RoP:D, p88) but that's about it.
Well, there are still plenty of infernal creatures that are not demons, at least I hope (I know that some theologians would argue that any infernal creature is a demon, but I hope this is not the case as far as rules are concerned). I would also hope that in their arrogance they are prone to let their nature slip from time to time. And wards work without needing to sense the demon.
Other than that, if you can trick them into a Divine Aura, their Magic Resistance will take a severe beating.
A demon turning into a spiritual form and running away is a victory, or should be. Demons shouldn't be the type of creature to wait a few days for the smoke to clear before confidently resuming their plans now that they've escaped being thwarted. That's requires a bunch of virtues that demons just don't have. The demon should only remain a threat on that plan if being attacked and turning spiritual was actually part of its plan in the first place - in which case you can probably also expect some gloating.
Demons that are thwarted tend to instead abandon their plans and either go sulk to their masters or come after the thwarters for vengeance. Probably slightly ill-conceived vengeance at that. They're demons, and demons don't do wisdom.
Supporting demons with diabolists gives a nice tangible target for any given story. The diabolist themselves can be promoted as the 'dark champion' of a particular plot, and can be squished by the players at the conclusion of the story. The demon pulling the strings is thwarted, and can come back at some point in future with a new plot and a new mortal minion.
As immortal/eternal beings, demons shouldn't be too easy to pin down and finish off - even the smaller ones. It should require some foresight and planning on the part of the demon hunter: wards and circles to trap the demon, PeVi spells with target 'room' or even 'circle' so you don't have to hit the demon specifically, etc. It is true that a magus can't target something he can't detect, but a magus has no trouble targeting a room or a circle that just happens to contain a spirit-demon.
With regard to Sense Holiness/Unholiness - I don't see this as an instant demon-detector, or it'd ruin far too many good stories involving demons. Instead it can spot the demon's handiwork - in the same way that spells are no longer protected by a magus' parma, demonic powers are no longer protected by its might once out in the wild. Sure you might not spot the demon, but you will spot enough clues to work out that its time to break out your anti-demon kit. Second sight won't tell you this.
As for your last point: I haven't read enough of RoP:Divine to comment. Of all the RoP books its the one I find the hardest to really get in to.
Halancar: ROP tI p28 : "any creature with an infernal might score may be properly called a demon, although a distinction is usually made between "true demons" which have made a rational choice for eveil and "infernal creatures" (...)."
"Demons are creatures native to the Infernal realm, although some have their ultimate origins in the mundane realm (infernal ghosts) or even the Divine realm (fallen angels)."
So every creature with infernal might also automatically gets the benefit of being able to mask their nature ? There is no such thing as a big bad infernal beasty with no brain worth mentioning and less subtlety, burning and killing indiscriminately ?
I think that those abilities are those of the "true demons" (demons SS), not the abilities of demons SL. It is explicitly stated for infernal ghosts, and I assume that the "infernal beasts" are also subject to that limitation.
That let the true demons, which are creature native to the infernal realm, IMO.
There are infernal creatures with infernal might which aren't demons ("Corrupted Beasts"). Even amongst demons they're not all clever enough to make the best use of their spiritual form... but those that don't are easily dealt with. It's the demons that do which are the most troubling, and there seems to be basically no way to deal with them directly.
From what I can tell, clerical exorcists don't really exist in Ars Magica. I mean, if you pray with someone you can perhaps drive a demon from them by virtue of a Holy Connection, but otherwise demons seem to have the run of the place.
Right, I don't like the RoP:I book myself, but so many people were telling me that all creatures with infernal might are demons that I had to to check it myself. It states, p.28
That would be the worst possible sentence to write. That book, and for that matter the core book, describe a lot of advantage of 'demons' without specifying any further: the inability of Hermetic Magic to penetrate their disguises (Limit of the Infernal), the fact that only True Names are an Arcane Connection, the ability to take Spiritual Form and in fact the lack of a physical body.
And then in the Corrupted Beast chapter you have wordings like "the beast has a weakness similar to those of demons". Not 'true demons', demons. So are they demons, or not ?
After a little reflection, I have concluded that the sentence I quoted above should be treated as pure fluff and not as anything that has an actual bearing on the rules. Rule-wise, the only demons are the 'true demons' described in chapters 4 to 7 of RoP:I, and they have all those annoying 'demon' powers starting with the ability to deceive hermetic magic. Corrupted Beasts, described in chapter 8, are not demons rule-wise (no matter what theologians call them), cannot deceive Hermetic Magic, do in fact have a physical body as a general rule and generate Arcane Connections to it, and so forth.
Seconded. Not everything with infgernal might is a demon. Demons live in hell and come to Europe to pay a visit. An infernal creature is similar to a magic griffin, or a divine unicorn: a creature associated with that realm, but not a demon.
Where are the rules for this? I didn't see them, but I miss a lot of things (sorry ExarKun!).
I know that a Relic can be used to make a certain area untenable to demons, but other than that, I really don't see anything that could compel a possessing spirit to leave a body unless you have an Arcane or Holy Connection to your target. Even Exorcise the Filthy Spirit doesn't work without a True Name, considering that the demon is in spiritual form.