Is there a level limit to how high you can have your main character get? (And similar for followers/slaves?) 

If there is I'd like to also know how to raise it a bit. To preserve sandbox play. And is there also a way to raise that level limit by changing something in a saved game file? Thanks. 

Yes, there is a level limit. Unfortunately, no amount of save file editing can change this limit. The only way to raise this limit is to mod the game's code to use two variables instead of one variable to count character levels. Though you would need to level-up non-stop at least 300 times per second to reach the level limit before the Sun makes Earth uninhabitable in about 1 billion years, so maybe it's not worth worrying about yet.