Believe is overwritten. I don’t speak for discourse this is just a guess.
With the discourse system am guessing there isn’t a log of previous e-mail maintained on an account when that is deleted/overwritten. However, to be sure with this specifically at whatever site you are talking about, would recommend that you ask an administrator there if they have a record of that or not. If they wrote that down or made a copy they will still have a record permanently unless they delete/destroy that.
You can make a request for full anonymization of a previous account and reference GDPR law, which mandates personal data be removed from databases within 30 days or so typically depending on your country if you or the site are based in the E.U.
Again still guessing but with an account merge that would combine data from both accounts.
You can request account be fully deleted but administrator has a right to maintain some data and public posts as long as that is in compliance with the GDPR policies for their country.
This seems like probably no, however if accounts are merged then you can edit out previous e-mail.