But 911 is not available in either situation.
And with T-Mobile being required to provide 911 calls everywhere they provide service, well, they do. They support VoLTE on band 12, whether the phone does or not. And if Motorola decided not to disable band 12, how could T-Mobile get in trouble for that? They have no control over the phone and they don't even sell it.
Just like you said, "the phone will find any network that it can and use." That would be like fining T-Mobile for not allowing a 911 call on a CDMA phone in an area where T-Mobile is the only carrier with service. T-Mobile supports 911 calls there, but the phone does not support T-Mobile. That's not T-Mobile's fault.
I honestly don't see the problem. If someone is in an area with band 12 and no other signal, they won't be able to call 911 whether band 12 is disabled or not, so why not at least let them have data?