At some point they decided that Support was for help with using a working Discourse site and Installation was for help for self-hosters who don’t have a working discourse site. It’s not help with installation, but help with your installation, whether it exists or not. So it’s installation as a noun rather than a verb. I was confused the first dozen times they did it to me too. 
You can’t. There’s no going back. (Unless, perhaps, you have a backup image of the whole server.)
I thought I’d seen that you’d reverted to some old server image and your site was running again? Or maybe that was someone else.
You can answer the above questions and maybe Michael can help you. I think that it will be much faster, easier and safer to move to a new droplet. If you have a recent discourse backup ,you can just do a fresh install and restore that backup there. If you don’t, you can follow the Move a Discourse site to another VPS with rsync instructions and try the upgrade there. You have nothing to lose since your existing site will keep functioning. If you find that it works, then you can stop your old site, run the rsync command, do it again, and then switch the DNS.