They still give all the code to their customers and as it is still GPLed code, noone can stop redistribution. So I'm wondering who will be the first RHEL customer which runs some "open mirror" of the RHEL codebase.
It doesn't. The GPL is satisfied as long as they provide you with the source code for the version of RHEL that they distributed to you. But they're not obligated to continue distributing later versions to you.
They absolutely can, but RHEL Red Hat will likely stop doing business with them if they find out (and thus stop giving them new versions), hence why they would only be able to do this once.
There's no restriction on distribution. You're free to distribute the GPL software you got from Red Hat.
They're under no obligation to ship you other, different software in the future. You're only entitled to get the source for the binaries they distributed to you. If they never give you the next version, you have no right to its source.
This is something for the courts to resolve, but it seems to me that there's a good argument to say that threats of future punishment (explicit or implied) would constitute a "further restriction" under the GPL.