@Ovi could you try setting this variable ^ again? Grist needs to know who the initial administrator is, and this is one way to set it.
For the question about guests, there’s a GRIST_ANON_PLAYGROUND that can control that functionality in multi-site operation. There’s also a GRIST_FORCE_LOGIN flag to require logins in all circumstances. See the README for details. I hope we’ll be surfacing and highlighting this behavior in the admin panel in the future, along with other security checks we’re adding now.
For the question about accounts. It is possible in a single browser session to log in as multiple accounts, and see all the sites you have access to. You need to be deep into Grist usage to care about this, and not likely to matter for you. From the README, I think you could hide the option by making GRIST_HIDE_UI_ELEMENTS contain multiAccounts, though I haven’t checked this.
Personal sites are a little space users have for documents that by default are private. The (Legacy) label worries me a bit. Thoughts @georgegevoian @jarek ?