I disagree. Isn't a big point of adding time constraints (something you're strongly in favor of) that now there is more risk-vs-reward? You can:
-be tactical, conserving resources to try to get through the Dungeon without long resting. In this situation, you arrive in time to save Mayrina.
-go all out in each fight, long resting 1+ times after entering the Dungeon. In this situation, you have taken too long and Mayrina is sent off.
With "Full Recovery Potions" added before boss fights, you're rewarding players who used up all their resources before the final boss and taunting players who have painstakingly gone through the fights trying to use as little resources as possible.
I'm especially against being able to simply buy these potions, unless they are prohibitively (>20k gp) expensive. If they can be bought cheaply, then any areas where you can't fast-travel lose a lot of their tension.
Finally, there's a realism argument. Why are incredibly powerful Full Recovery Potions (even worse if they are Zones) just lying around in areas before the boss fight? It makes sense in the tutorial level because it is the tutorial level. If these potions are found anywhere, they should be after a boss in that boss's treasure hoard.
