Not entirely, it only blocks certain unavoidable NTR scenes and the Yarizo relationship. You can still tank Kohaku's love stat and build up her lust stat to unlock some optional cheating scenes with her from random guys around town. You'd have to go completely out of your way to do it though, she starts at 100 love and 0 lust and it's no small amount of grinding to reverse that without cheats.
No, it has no impact on the MC's ability to cheat on Kohaku. There's a decent amount of cheating scenes MMC can do.
No, although Kohaku does have quite a bit of solo content where you're playing as her. For what it's worth though Jimmy is an incredibly under developed character to the point that he's virtually impossible to treat as a self insert. So you won't be able to complete quests and get through the storyline without playing as him, but you really shouldn't be identifying with him for getting cheated on or w/e. He's kinda just there.
