1)Is there a cap at how much affection u lose and is there away to prevent it from decaying? (Ngl taking care of all girls affection is annoying as hell I wish there was a quest to cap it at 100% and be done with it)
Not if you want them pegged at 100, that will always decrease a little bit every day or so even if you have all the house improvements, the perfume, a Pure slavegirl, max allowance, best stables etc. I have been able to peg some girls at 100 if they were Starlets dancing at the Golden Dragon and had all or almost all of the other bonuses (except I limit them to $25 allowance these days).
On the other hand it doesn't actually matter much if they're at 80 or 100, I just like to keep mine at 95+ most all of the time for roleplaying purposes. It makes sense to me to keep them all that happy you have to do a little work.
2) what happens when u max out poker fame?
Nothing as far as I know, though if you lose one night you'll find it won't stay at the max, you'll be a sliver off. I saw someone say that max poker fame was supposed to make it more difficult to win but I've not noticed that either.
3) what's the most useful religion?
For combat, AEsir worship, for finance Islam, for hunting Church of the Elder, for Influence Church of the Two Moons, for stamina Christianity, for willpower up to 80 atheism.