Initially I tried to use NIP-98 auth events for blossom. But it didn't work. NIP-98 requires that the full URL of the endpoint be in the event. Which means that the event would only be useful for uploading to a single server.
For blossom, the user can upload to multiple servers, similar to how nostr clients post events to multiple relays

I can't easily answer your third question. Since I'm typing this on my phone. But to summarize
NIP-96 allows media servers to describe themselves so nostr clients can easily integrate with them
Blossom does allow media uploads. But its goal is to allow blobs to "move" around between multiple servers similar to how nostr events move from on relay to another