Using assassins for expansion

[Festin]

There are several nations with good recruitable assassins, which, as I understand, can be used to expand through indies with relative ease. My question is: how exactly does it work?
1)How do I know, how many commanders do the indies have? If I assassinate one of two commanders and attack, will it weaken the indies?
2)Do indie commanders reappear? Meaning, do I have to assassinate them and attack the province with other troops at the same turn?
Any other advice is welcome too, of course