Wish I knew anything about the endings or NG+, whenever I play these sorts of games I kind of just dick around in a save until I get bored and make a new character with a new build.
Personally I find the idea of playing the default Demon Summoner a little boring so I have experimented more with the other classes. You're right, Adept is pretty cool, you basically make your MC into a customizable demon with their own level up tree. I've played that and Persona User, as well as a regular Demon Summoner once.
For Demon Summoner, my favorite stats are AGI and INT. I don't super heavy invest into making my Demon Summoner an offensive powerhouse with STR, instead I like to have him support the team with items and have his main offense be bullets and guns, the accuracy of which is affected by AGI. INT is actually pretty good as a Demon Summoner too for negotiating with demons. Have you ever noticed that sometimes even if you pick the "right" option with the demons, they still get mad? That's probably an INT fail. INT can also give you another chance to keep talking even if they get mad, and this can chain, so if you have a high INT it's practically impossible to fail a negotiation. This can, of course, make it easy to catch demons you want OR save your ass if you find yourself in a fight with an enemy you can't defeat. Also AFAIK it affects magic defense, so that's a big plus too. Aside from that, it's important to keep END from falling off too so you're not getting one banged.
Personally I always take the first girl as a combat ally before I turn her into a slave. You can always just decide to train her later at any time with basically no penalty, so there isn't any reason not to. Once you have more party members or strong demons to pick up the slack (and you have some more $$$ for training items to speed the process), you can then train whoever you want. You can turn an ally into a slave, but until the training process is done and they reach a fall state (Love, Lust, or Submit), a slave can't be an ally.
As far as cheats go... I dunno really. Personally I've had the most fun with the game not worrying about endings or anything and just playing it by ear. I don't think you should have to rewind too much considering each day has 4 segments, though. As long as you're not like, using Rest to sleep through days, it should be enough time. Make sure to utilize early party members (like your main girl and Pixie) and any special ammo you might have access to in order to get an early edge in dungeons. Realistically your goal is to be clearing those easy level 5 dungeons day 1 if you're coming at it progression-minded and go from there.