Not necessarily. Depends who bids for it, how much they get and what they have to pay for it.
Had the merger not occurred (with Masa failing to infuse any cash into the business), Sprint would be in an even worse position as T-Mobile could have picked some up, unless AT&T or Verizon grabbed it all. Sprint was already circling the drain. This would have finished them off in my opinion. Sprint didn't have the money for lowband, and it wouldn't have had the money for this spectrum either.
The efficiencies realized through merger synergies should allow T-Mobile to pick up some if it wanted to.
Personally I'd keep the Airave 4. All the femtos are the same when it comes to VoLTE. Your resets are definitely delaying it getting reactivated. I know it sucks, but you just have to leave it be for 3 weeks. Eventually VoLTE will get turned back on. If your S10s are on Sprint (check the PLMN in the engineering screen, or use an app like Signal Check Pro), then they will eventually find the Airave again. The network completely controls what your phone connects to, unless you lose service completely. You will not end up on the Airave unless the network tells your phone to connect to it. Usually it takes up to a few weeks for the Airave to get integrated into the network and for the towers around you to know about it. Once they do, they'll allow handoffs and will move your phone to it when it's in range. As to why the other phones see it and connect, there are a number of possible reasons. One of which is because it doesn't have VoLTE enabled (I've found that devices that use VoLTE will avoid moving to something that doesn't have VoLTE enabled). But basically it will be inconsistent until the network has integrated it. Every time you factory reset (ie, the PCI changes), it wipes this and has to start over again with integration.