1. Thresholds, if coming in uninvited:
* Blocks all kinds of spellcasting.
* Blocks other active powers(Glamours, Incite Emotion, Domination etc.)
a) Passive effects? WCVirgin, werecreature or changeling Speed/Strength/Toughness etc. Any effect when entering or inside? Any difference on are they always on or limited by shapechanging or feeding dependency?
b) Owing favour to fairie, unspecified when or what, fairy comes to collect and no other intentions. Does that count as invitation? Can that fairie do that stab-your-own-hand-if-refuse thing that Mab did at one point?
c) True Faith abilities. Probably can get in just fine since mortal, but using these while inside?
Sounds about right on the first note. You have to overcome the block strength of the Threshold to make a spell (or similar effect) work, but I don't think it'd fizzle glamours/dominates.
As for the passive effects you are asking about, I would either do a compel on their High Concept to keep them from accessing their powers, or use the Threshold as a "negation" of their powers; all powers reduced by the Threshold's Rating while inside uninvited.
I know in Ghost Story, it was noted that the White Court Vampire that walked into Murphy's house (which has a decent threshold) left almost all of her power at the door, so I think the above could work.
As for owing the favour, something like this was mentioned in Cold Days, specifically
when it was noted Cat Sith could enter a threshold uninvited for benign, good, or unharmful reasons, but if he wished to kill someone, he would be unable to do so due to the threshold
. You should also note that when Mab did her little harassment of Harry, he was in his office, not his home, so she was exempt from this rule.
Finally, True Faith is probably the exception to the rule, as it tends to almost ALWAYS be the exception, especially considering that it cannot worth when acting against your faith. So if you are in the home of an Evil Warlock, uninvited, you could probably let loose with your Faith Powers as long as you were trying to stop the Warlock from harming someone (and not using your powers to steal, for example).
2. Exact words trickery and fairies. How would you think about these:
a) "I am willing to consider if (insert favour)." After favour willing to listen but since no actual promises made, can just say no after some discussion.
b) "One favour from (plural) you." While pointing all of them. From fairy's perspective that is one favour from each.
c) If players try to negotiate about favour, promising not to ask anything dangerous for them. Then ordering, not asking, them to do something. Or just pointing out that according to fairy's judgement, favour should not be dangerous. Or just trading the favour(s) forward to another fairy that is not bound by first ones promises.
This whole section is a can of worms waiting to happen. Yes, they take it literally, but also remember that the Sidhe are EXPERTS at this, and it is rare when a mortal gets and advantage.
For example, for your first note, the Sidhe would be required to perform that favour, but that may not be done EXACTLY as expected...and they would probably do something to ensure that whatever they were asking for would be "required" anyway (like Mab in Summer Knight).
For your second, it could be taken as a favour from each one...or a mass favour that they all have to participate in. In fact, when one of those fae is asked for a favour, the whole mess of them could show up and make things much more complicated than expected.
This last one is all lawyer~ish. Negotiation is normal, but it is something to look out for.
I ask because while I want to drive home the fact that fairies think literally and differently than humans, I really don't want to be a dick.
When it comes to using the Sidhe, I have YET to see a GM not be a dick to get that point across.
Players have not read the books.
Isn't that usually the case?