UnityAssetFetcher is a library that helps mod creators resolve ingame dependancy at runtime (for Assets mostly)
Just add the dll(in release 1.1.0) to the BepInEx plugins, then add the dll to your Assembly Dependencies, just like a BepInEx plugin dependency !
Absolutely, feel free to use it inside your mods, the license may only allow for extentions, and to stick with this, it is extentiable ! you can customize importers, and even add your CustomImporter Each unity asset's type only have one importer slot, but you can add CustomImporters on an asset basis.