Update: my prediction was correct
They ignored my "request" for availability. So, I'll call HR before I go to sleep and, if nothing changes next week, I'll work the 6 hours I'm scheduled instead of the 8 hours they want.
The schedule? 4 on 1 off 1 on 1 off
The request? Basically any 3 consecutive days, I was flexible with them.
Ignore it? I'll do exactly what I have to do to not get fired, but inconvenience them until the request becomes a part of my schedule.