On the subject of keeping track of which factions you've had marriages with, I'd be interested to learn whether the game really does track the genetics of it so, for example:-

You marry a daughter off to the Danes;
A few generations go by and the Danes suddenly have a glut of daughters;
You need to get your heir married and approach them.
This is basically some form of cousin-marriage.
Another few generations pass...

Do your male heirs come of age with "chinless" or other varieties of "less than eight great-grandparents" vices?

There are more complex permutations, such as you marry to the Danes, a later generation of Danes send a daughter to the Spanish and a later generation of Spanish sends a daughter to you, and so on - but all with the same potential consquences.


The only strategy that comes to mind is that you adopt the following policies:-
1) Consistently accept princess offers from minor factions who you are unlikely ever to need to go to war with to win the game (60% conquest).
2) Don't offer any subsequent princesses of yours back to the factions in (1).
3) Consistenly offer your princesses only to the factions who you need to go to war with in order to win the game but from whom you will occasionally need to obtain a ceasefire - a respite, in order to rebuild troop numbers, tech up, or deal with another faction on a second front.
4) Don't accept any subsequent princesses back from the factions in (3).
5) When situation (3) is not applicable, use princesses strictly for surveillance and remember to bring them back home in time to marry your generals. 'Home in time' means that they get only about 10-12 years of useful travelling (which is why I prefer to rely on bishops).