Is it possible to change spouses once you marry? I have not seen any discussions about divorce so i don;t think that is an option but how about if your spouse dies? Or if you kill them? Does marriage become an option with another NPC again?

no divorce or re-marrige options

Nope, you marry for life, if your spouse dies then you become a widow.

Not without sneaky console commands (on PC version only), and no. Even if your spouse dies (whether you killed them or not) you cannot remarry within the confines of the game as it is.