A problem I found when working on MBS-13898 is that when adding or editing an alias and marking it as primary, we currently change the primary alias (dropping the flag from whatever is primary at the time) immediately on edit entering, even if the edit is pending, plus we never change it back to the previous alias if the edit eventually fails.

The current way of doing things doesn't seem to make much sense. There are two options that would make more sense:
1) We move the primary flag to the new alias, but remember the alias that was primary before, if any. If the edit fails, we reactivate the flag on that alias.
2) We keep the primary flag on the previous alias until the moment the edit is accepted, and only then drop it and apply it to the new alias.

A third option, I guess, would be to keep the existing mechanism, but just never make alias edits votable, even if the user asks for it or they are untrusted. Most alias edits already are autoedits since that is the default, and they are generally trivial to undo.