From what I understand, there is something in Verizon's agreement with the FCC about allowing any compatible phone on their network. It is all part of the deal that does not allow Verizon to throttle unlimited users, or to block apps, or to SIM lock their LTE phones. That's what I'm talking about.
Sure they can, just like any other carrier. All carriers have a blacklist.
Verizon could have blacklisted Microsoft phones. We have no reason to believe that they would have though. If the 950/950XL/550 had Verizon's CDMA radios, they would by default work on Verizon. Would Verizon have deliberately blacklisted them? I doubt it, but I do not know.
It seems to me that you are of the understanding that unlocked but Verizon-compatible phones do not work on Verizon by default. If this is what you think, you are incorrect. They by default work on Verizon, just like any GSM carrier. They would need to be deliberately blacklisted to not work.
Either way, it is a moot point. They are not Verizon compatible.
I think you're getting whitelisting confused with the phone actually working. The aforementioned factory unlocked iPhones and Androids that worked on Verizon were not whitelisted until recently. Whitelisting basically means that the carrier officially recognizes them, supports them, activates them, allow BYOD discounts, and whatever else it involves. Basically they are treated as the carrier's own phone. Any compatible phone will actually work on Verizon unless it is blacklisted. Since the unlocked devices were compatible, they worked even though they were not whitelisted. If you had an active Verizon SIM card, all you needed to do was to insert it in said device, and it worked. Lots of us did it long before our devices were whitelisted.