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.