1st of all i think you can marry other people as long as she doesnt find out
2nd You have to 'flirt' with her and use a different one each time and also give her a black rose i believe and she gives you Solus Greatsword i think it is.
3rd As far as we know your character doesnt die of old age and it age caps at 65 i believe.
Hope these help and welcome to the forums. Sorry for being vague etc had hard day at work
