I'd give you some hints, but I don't see any way to hide them behind spoilers tags or the like so that other folks who want to figure it out for themselves don't accidentally see it. Actually, there's supposed to be a hint cutscene for every stage which triggers if you fail it three times in a row (and you can get to it quickly by using the stage menu's Reset Stage command a few times). Does that not show up for you? If you really want I'll send you a private message with some advice. To encourage you, you only need to complete five stages from the second and third portal rooms to proceed, although you won't be able to see the golden ending unless you complete all the stages in the game.
Does the game seem laggy or stuttering to you? I tried showing it to my relatives during a Christmas visit recently, and despite the fact that the laptop we used can handle games like Undertale with no problem, it had jitters when playing Final Fancy. Seriously, RPG Maker MV games are performance hogs? c.ca Who'da thunk. Anyway, it made it much more difficult to deal with the time-sensitive puzzles like stages 7 and 12. If that's happening for you, I'm sorry, but I don't think there's much I can do to fix it. :/
Thanks for reporting the fireball bug. That's one I might be able to fix. Don't think I can do anything about save files from previous versions not working, though. Thanks anyway.