NPC means non-player character and most games have them. Seeing as how CK2 is a game revolving around characters and you as a player play one character at a time, it's very accurate to call the other characters NPCs.
I get the feeling there should be sarcasm tooltips in every forum.
besides, if you want to argue semantics, CK2 is primarily a sandbox strategy game with role-playing elements, just like King Arthur is primarily a linear RTS with role-playing elements. The RPG genre focuses on the overarching story. And besides, you can play any character in the game, which pretty much contradicts what "non-playable character" means
on another note, I never knew about the tooltip about demesne info until this thread :laugh:
still, back on topic. It really depends on how you want to play this game.
If you want to power-game it, then you could always load as your heir to arrange marriage, then reload back. Or, you know, give your heir all your titles except one, then reload as him.
If you want to role-play, there's nothing you can do with a marriage made out of love... or lust. Just make some story in your head to explain why your heir married that inbred huncback dwarf courtier
If he's landed then he will always make the decision himself. Probably the best thing to do would be to try to get a suitable female in your court and arrange the marriage between them before he does it himself.
Actually a bigger issue is what happens if the inheritance takes a long time (ie your current king stays alive). The heir will probably have children himself. If he dies then they will inherit his lands, but if you have more than one son then one of them will become heir (depending on your succession law). This means that all the land you gave to your heir because you expected to get it back into your direct control will end up in the hands of a powerful man who has a claim on your throne. You can never be sure who will live and who will die.
I depends on your inheritance laws anyway. If it's primogeniture, then you're pretty much guaranteed to eventually get back that land in one generation or another, unless he assigned a vassal to it (ie. he can have vassals and he went over his demesne limit, which means he'll definitely give land away).