WOW MAN, THANKS for all that info!! I'm blown away at how detailed it all is. From what you say, maybe I should just use this phone at home on Wi-Fi and just figure that I've learned my lesson not to do stuff that I don't fully understand.
From what you've said, since I have no idea who the previous owner of this Lumia 950 was since I bought it on eBay & thus I also don't know who had Unlocked it, there's no way I could do what you said to do in your TIPS paragraph.
If I really wanted to put all my effort into restoring this phone, I'd have it unlocked somehow (maybe AT&T would do it for me even though I'm a T-Mobile customer?) and that would probably cost me. Then I could do the Factory Reset thing or the WDRT thing
or even the OTC and get it to be the latest & greatest. But, let's admit, Windows phones are on their deathbed so why fight a losing battle? Put the thing in the bottom drawer hospice center or maybe just use it AS IS for a few months or more at home as a
toy to kill time on sites like Facebook etc. on my Wi-Fi.
After all, as I've said, I can see my iPhone days have arrived & I even luckily have my son's old iPhone 5-S to use to learn the How-To's of iPhone life. I can always dream of upgrading down the road.
We specifically chose to get a Family Plan for the 4 of us at T-Mobile since we knew we needed their SIM service & not CDMA for both my Windows phone fetish and since my kid's wanted iPhones and I'd been with AT&T for a long time back with my first 5 Motorola
MPx-220 Windows phones (those legacy flip phone smartphones) and their service isn't even yet still as good as T-Mobile's is up on this big hill our house is on that overlooks all of northern Orange County and even downtown LA.

I originally got a Nokia Lumia 635 from T-Mobile & them my first 950 before T-Mobile quit selling Windows phones.
When I fell into my pool with that, I had no choice but to buy an unlocked 950 on eBay or buy one on eBay & get it unlocked.
So, that's it. Thanks for your help. I owe you one. If you or anyone out there in this thread would have a hankering for this phone along with the big 9,300 mAh Mugen battery I have on it and the other Mugen battery that I was given in this thread for a
Lumia 950-XL, I'd make someone a swingin' deal. :-)