I don’t know about “alone”, but I think you’re likely to be in the extreme minority. The Discourse SSO route is the supported, stable way. There’s no guarantees that any cookie will keep the same format over time, so you’ll need to carefully test every update of Discourse to make sure the cookies are still in the format you expect.
I don’t think the SSL works quite as you describe, though: while it’s true that the browser will go back to the forum to get signed on, it should be transparent to the user. The main site will redirect the browser to the forum for the auth check, the forum will validate the cookies as being correct, and then automatically redirect back to the right place on the main site. The user probably won’t even notice, let alone care that there’s a once-off redirect chain to get them signed in on the main site.