So, it seems like the main catch y’re having is a failstate.
Now, I ain’t a dev nor do I know the fist thing about coding. Hell, I’m a screenwriter. That said I do dabble on game design. In a very amateur manner, but I figure you may be very well able to use a hand.
The way I see it, you got several options for a failstate. You might be able to use one, some or all.
-On the one hand, you got the KO states once an adventurer eats too much. You could utilize those by causing a monster attack to permanently raise the fullness of a character, to the point where, eventually, even the smallest attack would auto KO that character. In that case, the Priestess would work a double function: Recharge cooldowns and clear that perma-fullness (or part of it), gaining the weight that has been cleared.
The failstate would happen once the full party is KO’d. The state of perma-fullness would be cleared after completing a stage, perhaps counting for the adventurer’s weight if you think it’d be appropriate.
-You also have the possibility of utilizing a traditional, run of the mill, healthbar and damage. It’s the less imaginative solution, but it’s also effective and efficient. In this case, the Priestess would reset cooldowns, heal and gain weight based on the amount healed.
Failstate would occur once every party member’s health reaches 0. At the end of a stage, all damage would be healed and all characters would be revived. Mid-stage revival could be handled by the Priestess at the cost of enough weight gain to considerably slow her cooldowns.
-The thrid alternative, rather inelegant in my opinion, is to use timers and/or thresholds. “A stage has to be completed in X amount of time” or “your adventurers weighing X or less amount”. I say it’s inelegant because this is the kind of system I’d use for unlocks (which sounds like there’s going to be), however, some devs have gotten away with far worse ideas if you ask me. The Priestess would have no change to her mechanics like this.
The failstate would happen if you fail to meet the objective, simple as that.
Ultimately, that’s what I can offer. That said, you’re the gamedev, so you probably know better than I do what player behavior do you want to reward. If not, I’d recommend you gave it a whirl, since that very question is the one behind failstates in most games and may help you more than anyone in this thread might.
In regards to the UI, it’s a mess. No biggie. Most games of this kind have extremely messy UI design, which is not a big surprise. For the moment, what’s there is good enough since the game is not complex enough to need more. Once it gets more features, and thus more complex, it’ll require more stuff. UI design is complicated as all hell, so I’d recommend you focus on polishing mechanics first and then worrying about UI.