Longinus:

Piety can increase Magic Resistance, but in Longinus's case since you're saddling him with the status of killing the messiah, that'd cause a degradation. This is doubly true since he's in the Lancer class rather than, say, Saber or Ruler.

With curses I mean:

[quote=Mahoyo]In this modern fantasy setting, curses can't be broken. A curse that has already taken effect will never disappear. The only way to release yourself from it is to transfer it into someone else. Thus, the term "Kaiju (Transfer Curse)" is used. It's not a misspelling. Kaiju (Release Curse) is a rare trait on the same level as demon exorcism. Since it entails erasing human destiny, well... to be blunt, it's the Mystic Eyes of Death Perception.[/quote]

Curing a curse is something on the level of True Magic, so you can't do that. Curing poisons and similar status ailments? A-Okay, but curses are another animal altogether.

Mitsuki:

Neither Shirou nor Kiritsugu have their Origins awakened. They are aware of their Origins, but that's different to having it awakened. 

You can have her be aware of her Origin and maybe have a Mystic Code or a spell aligned to it if you justify it properly. Something like the effects you've given her is a no-no since that's the sort of capacity she wouldn't have unless she's been researching and training for a couple centuries.

It's fine that she's thirteen, I'm just a bit baffled by why she'd leave the Clocktower and fly to a deathmatch in Japan at that young of an age. Like, she will have to be directly involved in murdering people and has very high odds of getting killed. It's fine, she can be hella irrational, but yeah.

She's still using it for transport, so I'd recommend using another method to avoid a Touko lawsuit. 

In that case, clarify what her Herbalism and Alchemy kits contain as far as what she can make with them.

On healing, clarify the method of healing, like does it just speed up the body's natural regen or what.