What happens when a Red Hat Developer Subscription expires

Short answer, they legally can't do that.

Most of the code in RedHat, like all Linux distros, is GPL-licensed, including the kernel and most (all?) of the core utilities. They can't release it under anything but the GPL, and so long as they distribute the binary they have to distribute the source. That also means they can't prevent anyone from cloning it (besides the trademarks, of course). That is one of the big points behind the GPL.

The "precedent" you specified in no way constitutes not releasing source under the GPL and has absolutely no impact on clones. They just didn't release the patches individually, they did release the entire (patched) kernel, making anyone who wanted their patches (in this case directed at Oracle, it seems) put in a little extra work to do the diff themselves and sort out which code changes go with which fix.

As mentioned in the previous question, CentOS is your best choice since it is derived from the sources of Red Hat Enterprise Linux (RHEL).

Also as mentioned in their Technical and Release notes, almost all your required tasks are met in CentOS.