I set my test group to require hashtags.
In another, not-logged in, browser,I sent an email without a hashtag in the subject to my test group using my non-moderator test email address.
(1)Since my test email address had not yet registered a password, clicking on the link I was sent to edit the subject, resulted in being redirected to the Please Sign In page. This is correct. I clicked on "Forgot password or don't have password". This went as expected. However, after I had registered I was not redirected back to the page for editing the subject and I could not find that page from the menu selections available to me. My suggestion is to, if at all possible, redirect back to the editing subject page if the users had to register before editing. Either that, or make navigating back to that page easy and obvious.
(2) I eventually went back to the email and clicked on the link in it again. When I started typing in a hashtag I realised that I didn't actually know which hashtags were available. and there is no dropdown on that page showing me what existing ones are. Is there some combination of group settings that does make a hashtag dropdown or something similar visible? If not, my suggestion is to provide some mechanism on that page for the user to see available hashtags.