Agree - I think that I’ve pinned it down a little more though. This functionality used to work, so this seems like a regression. Probably at the time of the fix alluded to in those other threads.
It would likely be appropriate to merge them all together.
Personally I believe this to be a highly valid use case. It permits the use of a single streamlined process to onboard people regardless of whether they are already on the site. Otherwise you need two parallel (and difficult to administer) processes for current and new users.
It may also be worth a ping to @gassim and @UnicornX as well to see if this has solved it for their use cases, and round out those topics too.
I’m getting an ‘invite expired’ on the one in Join the Discourse User Testing Community, which is encouraging as that sounds like it would work again if the invite redemption was extended. (Though that might also be because I already used it ages ago and forgot about it )