So I decided to marry Ysolda, who I had absolutely no idea turned out to be 'essential'. I have no clue as to why Bethesda chose to make random npcs immortal and others not. Such as why is Mjoll the Lioness immortal but not Faendal, or Ysolda but not Cosnach? Point is I don't want to be forced to have Ysolda as my wife forever, is there anyway for me (not on pc btw) to get rid of her as a wife? She isn't a bad wife, but I don't like the idea of her being permanent regardless of what I want.

She's needed for a few quests. It's not random. Most essential NPC's are related to a quest or something, so if they die you can't complete whatever quest they are related to which can be a real bummer.

You'll have to go back to an earlier save and even if you could kill her, you'd need a mod to re-marry with that character because you can only marry once with the vanilla game.

Damn thats awful..but as far as you are saying with the quest thing and them not being random I disagree. Faendal is not essential and he has a quest, same with erik, roggi knot-beard, annekke, amren, etc etc. Also, if they die during the quest, its just quest failed and you can't do it. In addition, even after you finish Mjoll's quest and Ysolda's quest they are still essential despite not being needed for a quest. I still don't see their logic for picking immortal characters sometimes.

I can see where you are coming from. I never noticed so many NPC's were "essentially" set wrong or at least it never bothered me enough to affect my enjoyment of the game, although I have made bad marriage decisions that I regretted and had to live with it or go back to an earlier save.