Find some old threads that you commented on years ago; chances are that a good portion of them are still there. Your reddit history doesn't show everything, and AFAIK there are no tools available that can effectively eliminate every single post you've ever made, unless of course you simply didn't make many comments and posts to begin with.
Nope. Reddit orphans posts and comments from accounts. I don't know the hows or whys.
Last week I stumbled on a 15 year account who tried to do a full wipe before abandoning reddit. Their last and only comment in their profile is their farewell message. Except when you Google search their username it shows their content is still there. Just not associated to their profile page anymore.
Request a GDPR export and you will get your entire comment history. There are tools that can read that history and delete all the comments. I did encounter errors deleting a small number of comments which may have been due to the subs being privated. I deleted the problem ones from the json and the rest got deleted. I manually checked a sample of the comment urls to confirm.
Delete your account, look again in 3 months to see if account is gone.
If not request deletion via email amd if not followed by a satisfactory action, file a gdpr complaint. That will fuck them over a bit.