There are three distinct chat servers attached to Stack Exchange sites: https://chat.stackexchange.com for the entire network, https://chat.stackoverflow.com for Stack Overflow, and https://chat.meta.stackexchange.com for Meta Stack Exchange. To chat on each server, you will need at least 20 reputation on the parent site.
There is at least one main chatroom ...
This is fixed.
The problem was a bad abstraction in the audit log code. The audit logs take an IUser abstraction when recording who was affected by a change; this can be a user, a room, or a message. Users and rooms both have concrete primary non-null Ids. Messages have nullable foreign user Ids that get nulled out when a user is deleted.
Your diamond on chat was removed because your chat parent was set to the site where you stepped down and no longer had one. Changing the parent to your Travel profile fixes this for you and gives you back a diamond in chat. Any moderator should be able to do this for you and manually refresh your profile, assuming they know to do so.
I've never heard of a one time, 4 hour link being possible.
I think that comment is talking about two different things.
"We're closing this loophole" meaning that room owners can no longer grant write access to people with less than 20 reputation, as described here. The answer is about room owners being able to grant access, so that part of the ...
It's a bit confusing
Chat.StackOverflow is for Stack Overflow users with more than 20 rep
Chat.StackExchange covers most other Stack Exchange sites and requires 20 rep across the whole network (includes SO)
Chat.Meta.StackExchange is just for Meta.SE and you need 20 rep on Meta.SE (this site)
"Visible room" is the one you see the messages of (and whose ID is in the address bar). This might be on a different tab (though your browser settings may control whether you hear anything from inactive tabs or not).
"All my rooms" are all other rooms you're 'in', i.e. the list between the avatars and the starred messages:
Of course, all ...