1. The event that grants the Master Seducer trait is called WoL.99 and is found in the file /events/wol_seduction_events.txt. There does not seem to be anything in there that distinguishes between males and females, except the nicknames that are handed out upon gaining the trait. This is probably just bad luck but do keep an eye on it - sometimes a simple save and restart fixes these things.
2. I am not sure if this is limited or not. I have on occasion had my character seduce and marry his dead father's second wife (ie his stepmother), I haven't tried with a biological mother. Try playing Zoro, I hear that they are quite "liberal" in this regards !
3. I am not sure but I think that most randomly spawned (as opposed to new arrival) Jewish courtiers come with the "true_believer" modifier which makes them immune to conversion. Try getting a new arrival (not event spawned courtier) to become your bestest friend (ie +100 opinion of you) then ask him to convert, if he can't won't then it's probably hard coded.
4. All sorts of things !
First off, if you join the relevant secret society you get to see which of your courtiers and vassals are secretly practicing another religion.
Second, if you are looking to convert your kingdom then you can start said secret society yourself and then work to bring in converts who will then go out and do your conversion work for you. Once these conversions start to snowball and you reach critical mass you can then go public and have a very good chance of not being pubstomped/excommunicated/killed in a religious revolt. Don't ever join a secret religious society that has been started by someone else though unless you can take over pretty quick, otherwise you will be subject to the leader's decision as to when to go public which will almost certainly not be when you want it.
Third, practicing in secret gives you access to religious specific benefits without the hassle of actually converting. The best known example is playing as a Catholic Crusader or Orthodox Doux and secretly converting the Shia Islam, which gives you the ability to join the Assassins !
5. Build deep for now, looking to expand once one or the other of the two big blobs starts to fracture. Don't hesitate to take small bites out of each as and when they are distracted ,e either with decadence or succession revolts (Abbies) or internal faction revolts (HRE). Sometimes just staying alive is a win, at least until you have built up your levies, demesne and retinues to the point where you can kick serious ass in holy war or ducal claim wars.
Just gotta be lucky and patient.
