1., 2. 3., what everyone said so far.
4. As for the DLCs, it depends on your difficulty excluding the Gold, EXP and Apothesis as they're preset. From what I could tell, Rouge and Redeemer 3 had different stats from Hard Mode and Lunatic. For example, Hard Mode in Rouge and Redeemer 3 had most enemy with at least 1 or 2 max stat cap while in Lunatic, most of their stats are capped. So basically harder your difficulty and more Stars it has, the harder it is. Well this is just from my experience. I guess there's also the Future Past ones as well but those should be done at end game/near end game no matter the difficulty or you did a lot of grinding.