mixintos only work for json files ATM. If you want to modify other files, override is required, though really really not recommended (though I’m also bad in that regard >_>).
So if you feel that you have to override, I’d say: look into Jelly. It extends the API in SH and can easily be modified for what you need, like overriding a LUA file to insert maybe a function or a trigger somewhere.
If you’d like to know more, look in here. RepeatPan explains it all in greater detail (not only in that post but also further down in the same thread).
Yeah it’s not gonna that easy unfortunately. I think that it’s only the max health you’d have to modify together with the level (I’m not completely sure on this though, just a hunch).