Indeed, in your case acquiring uBlue through its ISO was probably the best option; but I’m glad to hear that it worked out in the end!
Anyways, doing it the hard way is helping me learn the intricacies of an immutable system, so I am having fun.
Well said!
Just in case; consider the following:
Pin your current working deployment with the aforementioned sudo ostree admin pin 0 command. After which it remains accessible regardless unless you unpin it later on. This should allow you a working deployment if all else fails and thus a safe haven to rely on.