Third option for sure!
I always play "Ironman Mode" in every game that allows it (XCOM, EU IV...) and i force my self to do so in games that dont have ironman like Hearts of Iron, TW and more but i still can play this way if you can save at every point with multiple saves. But people that dont like to play like that should not be forced to do so. Best case - give a choise at the campaign start if you want to be able to have multiple saves or not.
Please do NOT limit the number of possible campaigns running! That would be the worst case imo. For example when i game comes out i imagine i will play a single and a multiplayer campaign at the same time. You could say one as CS and one as US but most likely the single player will be over sooner, so i will want to play the other side as well.
But theires not just that mp issue, maybe a friend of mine wants to test the game when he visits me or more than one person in a family wants to play the game or just someone wants to play a hard and a easy campaign at the same time for some reason.
Overall: Give the player as much choises as possible! If someone wants to abuse the savesystem in singleplayer, let him do. I and most people wont but who cares?