the multimc zip is not importable with any multimc based launcher, this is a mild annoyance that probably should eventually be resolved.

the main difference is you dont really want to override the instance config, i feel like that can be mentioned in the guide on how to do it since you have that, but i actually agree a separate zip is better if you can package the forge mod for lwjgl3ify in it

thats not an importable modpack. just some files you will need beyond the jar.

its trivial to make it one all it needs is a instance.cfg why shouldn't it be one instead of wasting the users time in having to create and instance and manually copy the files?

The multimc zip is intended for migrating existing packs to lwjgl3ify instead of making new ones, but I could make another one that's just a blank instance and importable. Afaik there's no way to make one that works as both