I was wondering if it would be allowed for me to let "teammates" or "testers" login to my main account (the one I use to login to the WebEditor, not for logging into the forums), so they can directly play-test my game throughout production. I also thought it would be neat if someone like my brother, or my cousin (who is actually a professional web-designer) could make/publish their own games under the same "company" username, where we would each include our real name under author for the games we make; while Quest is pretty simple to use, it might also be cool to have more than one person actually login and work on a single game project. Normally, I would assume that was a big no-no with any website, but it seems like it would make sense here, and I can't find anything that actually says it's against the rules.

Can I do any of that? Or do I just need to stick with publishing the game as unlisted for people to help me test it?
If that is the case, I'm also curious how it will work out if I publish a game in mid-production and then need to publish it again later. Will it automatically overwrite the outdated version, or will there be multiple instances to clear-out and delete?

I think it would be good if more than one user take on a single project. As for letting them use your account, that's your choice.

You can "update" any game you upload, listed or unlisted. It overwrites your previous file, so there's no clean-up required. You'll see a link for it when you upload your game. It's what I use the "unlisted" option for (beta-testing).
And as sonic pointed out, there are no "rules." You can make games however you want, it's a community to show what has been made, not an ongoing competition (unless you work better that way). No one will complain if "TriangleGames" turns out to be more than one person. We'll be all the more grateful for high-quality games!