But I guess after all that work it won't work, since there's no field for name and email like in WP comment form. And a forum system I assume forces visitors to register first without exceptions?

If you enable "Can add topic" and "Can post reply" permissions for Read Only usergroup the username and Email field will be automatically displayed for guests:

Cool 🙂 Yes, just what I was looking for. 👍 

Do I need to or can I disable guest registrations after enabling guests to participate from the frontend without being logged in?

I think you can. But the social login will not work if you have social login buttons. However all guests will be able to create new topics and post new replies.

OK, tested it and it works. However, even though I set settings to allow attachments from 1st posting of a new user, this frontend guest posting does not support adding a file. Even with advanced uploads addon.

How would I add this support for files please?

I'm sorry but I don't know. The default attachment is allowed, but I can't say for the addon.