1. courtiers are the useless bums hanging around your court. I't is from among them and your direct vassals that you can choose councillors and army commanders... you can also declare war to press the claim of one of your courtiers, so that he gets the land if you win the war (he doesn't always become your vassal after that though). you can also give them land to make new vassals
1.1finding wifes for most courtiers is a waste of time in my opinion, only do it if they have good inheritable traits such as genious.
1.2 well, think i answered that.
1.3 depends on where you are. if you're in russia, probably not. a bit closer is usually better. also, granting him land is uneccesary to press his claims
2. means that you are going to split up the realm when you die (either passing out equal titles to your chuildren in gavelkind or someone else is winning in an elective) or one of your vassals have an heir (or pretender, the two second closest to inheriting) that for some reason will inherit the land without joining you realm (could be a landed vassal of someone else or have an equal rank already)
3 this is a bit tricky sometimes, unless you go on a killing spree. your sons usually have a fair chance of marrying a landed femaly normally (at least the heir) but even then, it's rare. mostly, you have to marry someone whos is 4th or further away from inheriting, and the hope/arrange for the other threes death... then, if your heir does marry and eventually his wife gets a title, then his sone will eventually get that title and you will eventually be your heis and then his heir. (this is the basic, there are some more or less gamey and advanced versions of this)
3.1click the two ring in the character profile view. if they're not there, the character isn't under your control (ie, one of your courtiers)
3.2 not really. you lose some prestige (marrying down) but a good wife/alliance more than makes up for it imo. also, if it's you or your heir, you can always earn 50 prestige from the marriage
well, correct me if i missed something in inheritance as i'm hardly a pro at that except as a ruthless mass-murderer
i've killed so many kids in this game, it's kinda disturbing