I just started playthrough 2 so I haven't unlocked the outpost warps yet, but is it possible to switch back to playthrough 1 if I download the Madd Moxxi DLC and level up my character someone, then switch back to playthrough 2?
you decide which playthrough you want to play on before you start playing. You have to save and quit if you want to change it to the other one. (so it's not like some games where you can press start and change the difficulty)
However, if you're on playthrough 2.5, you can't go back to regular playthrough 2.