I can confirm your guesses. The atlassian-external scope isn’t “real”. Apps cannot declare it, nor can customers authorize it.
I can also confirm your understanding of the security contexts. It’s a known constraint and one we are trying to expand. But there are many directions to go: cross-product (same site), cross-site, org-level, etc. We’re still trying to work out the intersection of what’s possible and what’s valuable. I don’t think Teams would be unlocked anytime soon.
Agreed! And I’m glad to hear you’re thinking that way and that certainly lines up with how we want Teams to be used. But it could take some time for that become real.