In a non-magic peril encounter, can you use a mana potion to get magic dice and then use a magic skill to gain a heroic die? Can you use the magic dice to fill the grey challenge box for the dungeon?
Could you use a mana fountain skill to do the same?
In a peril encounter you are not able to use skills until after the dice have been rolled. You are only allowed to roll dice of the corresponding color of the peril check (and heroic/black dice as they count as any color). So unfortunately, no. You wouldn't be able to roll any blue dice and convert them into heroic dice.
If a hero has three health capacity but has one health left, and then takes 3 damage, do they reduce their health to zero, then take the healing potion, and end up with only one damage? Or do they have to heal from the negative health balance?
From my understanding when you take a potion to try to prevent death you would either take the full force of the attack, going down to -2 and then heal some back, or the hp gained and the xp lost would cancel each other out, resulting in you taking 1 damage. In either case, in the listed scenario the character would be at 0 HP after the fact and would be defeated.