Sorry to hear that. The fix for the NO MP situation is coming but, I'm sorry to say, from Episode 06 there's no more issues with the save files. As stated in the update post, you just need to use the update function in the Scroll Menu to fix your save file and then you'te good to go.
I guess you might have missed that post update? Or, as Episode 06 is currently only available to Patrons, maybe you didn't have access to it? :)
About the other elements, though:
1) that's demonstrably not true, other elemental attacks that are not the resisted or nullified elements do damage, just not as much as the the element an enemy is weak to (obviously);
2) the fight system is built around that. It's inspired by Megaten games where, depending on how prepared you are, fights are either long struggles or are over very quickly. There's plenty of dialogue hints at what's coming your way, especially in the latter half of Episode06, and you can change David's abilities on the fly.
You have a legitimate greviance about the MP Turn Limit. But about the other issues: the new save file thing has been fixed, elemental resistances work as intended, and battles will go well as long as you pay attention to what goes in-game and go in prepared.