Apple has put me in a pickle. I had a lot of parts (battery, display, logic board) of my MBP mid-2015 replaced at the local Apple Store. When it was returned to me, it had a big new problem: it would keep booting into internet recovery mode (although it eventually did boot), but eventually ended up freezing while I was using it. Trying to reboot into recovery mode put me in the same internet recovery cycle
However, it turns out that my MBP runs fine from a bootable external backup SSD over USB. Since running Disk First Aid at that point found no errors on the internal SSD, I thought I would try to reinstall Mojave from the SSD, but when I did so, the installer refused, informing me that my Mac had an incompatible firmware version and suggesting that I try to install Mojave on a HFS+ disk! (alas, my MBP's flash drive is formatted as APFS encrypted). It was then that I realized that what must have happened is that the replacement motherboard has an earlier firmware version that is incompatible with OS X 10.14.1 installed on the MBP's internal flash drive, and the so-called "geniuses" at the Apple Store didn't think about this.
The problem is this: how can I update the logic board firmware so that it is compatible with Mojave? I just tried an internet recovery install of Mojave, but the installer refused for the same reasons as before (incompatible firmware). I can't even see a way to downgrade the internal flash drive to HFS+ …
Is my MBP able to run from the external SSD because it happens over USB, or because that SSD happens not to be encrypted?