To be honest I will most likely have them just add normally, like 5% + 5% gives 10%, it will probably be easier to balance in the end.
There are curative spells in game. There is White Magic that can be learned from a traveling Priest NPC, you have to donate 1 whole Gold (10000 CopperCoins) in one go to him though. He will then give you a Skill Manual that teaches White Magic.
There was also a pet, a Snowman that had a scarf head accessory that allowed casting a restorative spell, but that was only available during the Winter event.
Water spells will likely be combined in the future with Ice Magic rather than having a category of their own.
Glutton Mechanic is a bit more complicated than that. Eating a food identical to your last meal has a 10% chance to increase your Gluttony score by 1. Eating an Unhealthy food decreases your Health Score (not Gluttony Score) by 10/20/30, similarly, eating a Healthy food increases your Health Score by 10/20/30. If your Health Score (which starts at 0) reaches 100, your Gluttony Score gets decreased by 1 point. If your Health value decreases to -100, your Gluttony score increases by 1 point. Once your Gluttony Score reaches 100 you will get the Glutton Flaw.