I set my main players to level 30 on their side properties bar and gave them insanely overpowered weapons when I needed to quickly skim through the fight sequences to test the story. This does get difficult when you have as many branches as I did so unfortunately my game still has flaws. In editing mode, you can also speed up a story run by pausing the game and teleporting the players to the next location (though this doesn't work as well when the game is close to completion as the intro screen messing things up and triggers become important).
If you want to test the difficulty of certain mobs, you could make an extra area or use an empty one, and put down the mobs you want to test in different sections so you can run from one to the next and see how hard they are. Drop a few potions or a bed so you can restore your HP to full after each fight.
Undoubtedly you will also miss things that other people will find since different people play differently. I had three other people play my game to watch how they would break the game with their own playstyles. One was just trolling and trying to break everything intentionally, which is called chaos testing and actually helps.
Helpful Programs: Asana helped me organize my tests and keep track of results, such as what still needed to be fixed and what worked. Basically, you can create lists, prioritize items, make notes, and check things off when they are complete. I also found something called a Cheat Engine for D:OS which may help since the person scripted in a godmode and unlimited action points etc. I've never used it myself though.