Hey guys, if I make a new game on a new account and a new PSN I can still switch back to my old game without losing it, right? I am not interested in new game+ because the whole challenge of the game will be lost, as it essentially already is gone on my main save

You can just put a save on a USB, I believe. NG+ is better than you're giving it credit for, though, from what I hear (not there yet, myself).

I just wanted to make sure it creates a second save on the same playstation when yr using a second sign in and a second PSN, I already have 1 of each of those coz of Monster Hunter Portable 3rd HD.

It has nothing to do with not giving NG+ enough credit, I know exactly what it is and I wont enjoy playing the same game in the exact same difficulty at lvl 80 with all of my stats AND gear. The main benefit of NG+ is that you can buy portcrystals, wont help with the fact that ill be 1 shotting most things :(