Intersting thing you can do is marry your princesses to the best generals you have. Better yet - choose absolutely the best one there and marry him to the youngest princess in royal family. When your king dies (provided you haven't been exporting your princesses abroad to marry into any foreign faction and there are no royal heirs available) the general in question has a major chance of controlling more valuable lands and better units in the oncoming civil war. Gives the campaign wicked flavour and makes it more real-life.

One thing that really gets me is reappearance of eliminated factions. On the whole I don't mind but several times the English and HRE reappeared with their king being 58 and the youngest heir about 50!! Not one of them a married man. I let them die out slowly, and yet, even after ALL the heirs have passed away aged over 60, some twenty years later they have enough cheek to respawn again!

Heir stats only come into play when your sons are mature. The moment they turn 16 they are given their first v&vs. Any vices or virtues are only given to the generals visible on the map.

Influence does help forging alliances but there are some other things that affect your chances.

a) faction profile (expansionist, defensive, merchant etc.)
b) emissary valour - the higher the better
c) your current position on the map.

All these statements come from personal experience and have no scientific background so feel free to correct me. Certain mechanisms seem to be fairly constant and after a good year and a half of playing the game (I will never stop having fun with it) I repeatedly get similar results.

Never encounterd any bugs in the since installing VI. No crashes, nothing.
It might be your system, mate. Try and free up as much memory as you can spare, close down any applications you don't need and see how it runs.
Good luck.