Ok, perhaps I can shed a bit of light to help out. Before I do that, though, I should preface that I jumped back into the game and completed the demo. I chose to go with a "dive in, clear a few required fights while running from every random encounter, leave to heal then repeat" approach. This did result in me getting my gear improved a bit along the way, but even when I got to the point of clearing the demo I felt like encounters could still be rough.
So everything I detailed is just how everything compounds onto the difficulty. From the high player miss rate to how strong enemies are to the low RM gains outside of specific treasures to the random resistance against spells, view everything I said as a total package. Not everything needs to be addressed to improve the difficulty curve. It just requires the right changes.
For example, if the player miss rate was lower and monsters were a bit weaker (such that it wasn't possible for the enemies to focus one character down and kill them before that character ever got a turn. Yes, this happened quite a few times for me, even moreso when I hit the surgery wing in full), that alone would adjust things enough. A lower miss rate would mean the player is hitting more often and defeating enemies faster, meaning enemies drop faster and players aren't taking as many attacks. Enemies doing a bit less damage would also mean that players have more health going into subsequent fights, which encourages them to keep pushing instead of going back to heal every few fights. Alternatively, boosting RM gain could be done instead of weakening the monsters, meaning players can choose to grind a bit and get some gear to improve their stats.
In short, it's not about improving everything. The reason I got into the RM grind thing a bit more was because of the counter arguments you were making to that particular area, but it's not something that matters if other areas are adjusted.
Also, I plan to do another run through the demo on my own to try out different classes (and to see if there's other feedback I can give). As such, is there any info you can provide about the different classes that may not be obvious (things like jobs intended for front/back row or ways to use stat modifications to change those intended rows)? Just any sort of info that would be helpful to know.