How does voLTE and Band 12 work for T-Mobile and can Cyanogen/AOSP allow us any control over this?
So I'm going to answer as someone who has a non-VoLTE phone in a market that has T-Mobile LTE, but no 2G or 3G coverage. 

With an alternate ROM (OR a way of editing the modem settings or keeping an older ROM installed) on your Android device, enabling Band12 will be no issue. It's possible with a bunch of Android devices already to enable bands that are available, but disabled by the OS.

What you'll have happen is exactly as described. You phone will get data service, but you will be unable to send or receive calls. You will get SMS and MMS messages just fine. My phone (Nokia Lumia 830 RM985) doesn't support all of T-Mobile's LTE bands, and HSPA bands. This means that in the market I'm in right now, I can't make or get calls as long as my phone is connected to a t-mobile tower. I can see when a call is trying to come in because my service will drop and my phone will roam, but the process takes longer than t-mobile's system allows and it times out first time.

Same with placing a call. First attempt times out and is registered as a dropped call. Second attempt drops to roaming and the call is placed.