Thanks for letting me know! The healing items are actually not supposed to work on your undead units, but if they don't work on the Prince, then that's certainly a problem. As for the cannon bug, that's an unfortunate limitation I've found within the engine so far- it can reduce an enemy's HP to zero, but a skill has to be executed on it for it to "count."
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Ah, saying it now sounds embarrassing, but it was a short-sighted design on my end. After the "First Act," you're meant to be able to recruit human units, which you can actually heal using potions and the like, but as you say, it is extremely misleading to the player to give them potions when they can't use them in the beginning.