This is a limitation of how fabric merges all mod datapacks into one. It might be possible to implement, but probably won't work right now
#packaged datapack mod template that reads overlay section of pack.mcmeta
kindred zodiac
you could also use a mod like paxi that will include all the datapacks you put in its folder in every world created for that profile
leaden musk
Yes. You could also just make a mod that loads a regular datapack from inside of itself, but that requires code. Packaging a datapack as a mod by adding metadata files doesn't require any code and is thus easier to do