If it does, I should be able to post images on the pixelfed app that show my username as @adonis.
It cannot work as stated because there could be another @adonis accounts in other instances and the only way to prevent that would be to centralize all the signups which goes against the whole idea of decentralization. That’s why the user must be @adonis1 as it is shown now.
Regarding the OAuth/JWT, again… not an expert, but what I understand is that that kind of integration is much stronger than the current system. AFAIK, it could work as you say, but that would make things much more complex for the servers; you usually provide OAuth authentication for a few services, I don’t know how well that scales with … hundreds / thousands (?) of authentication provders. But, who knows, maybe in the future it’s implemented in one way or another.
We should take into account that this technology is fairly new and people are still building on it.