Do options exist, such as,
-- UA is employer's preferred airline
-- OP is at a UA captive hub
-- or OP's destinations are only served by UA
-- ...

But then option of another star alliance program exists. Not good for domestic heavy traveller but better than nothing.

As some others have pointed out, there's nothing preventing the OP from buying UA tickets and flying UA in any of the above situations. UA didn't fire the OP as a passenger - only as an MP member. If mileage accrual is the issue, then joining another *A carrier's program and crediting UA flights to it would work. They could even be used to redeem award travel on UA. What will be missing is any kind of elite benefit, or the ability to get a UA credit card for benefits or join the UC (since both of those things are tied to the MP account).

He posted a UA review this week on a day time TATL flight ex US

It would not be stretch to speculate one or more upgrades may have been purchased by a representative of United posing as a buyer.

What’s a chase shut down? I’m guessing manufactured spending or something like that?

People seem to ask this every time.
He's banned from MP, not from UA.

Could be for that or providing fraudulent income information, too. We've seem a number of posts the last few years where members inflate income to get credit cards, and then when asked to provide proof of income come up with all sorts of creative ways to try to work around not having supporting tax documentation, such as it being planned income or family money given to them.

Slightly OT, but a quick eBay search also shows a ton of United Club one-time passes for sale. Back when they were the card-stock versions, it was easier to have a "buddy" buy or sell them for you. Now, with the E-passes, I would be extremely hesitant to sell mine online. They are definitely linked to you because I can see my usage history (last 2 were used in SFO on this date, the 2 before that expired on this date, etc).

I would probably be hesitant to buy then online as well, since I'm sure they could tell that they aren't mine when I scan them (although they have no way of knowing if I bought them or they were gifted to me).

If he obtains and maintains XX*G, where XX is not UA, he'll enjoy "complimentary" UC "membership" and a selection of other elite benefits on UA.

I very much doubt that’s the reason.

I don't think UA banned you from flying them, did they? I think they simply said you can no longer be a Mileage Plus member. Also, I wonder whenever I hear of someone deciding to sell a membership benefit rather than use it, then perhaps that membership isn't offering much value to them in the first place.

What did he do to get banned?

Sold RPUs or GPUs

First name, last name and DOB are not that unique. However, UA has access to additional data like passport numbers, credit card numbers, etc., all of which are far more unique. If the OP is willing to get a new address, new credit cards (or use some of the alternatives), and a new phone number, for their domestic travel only, maybe they could skirt by?

It really isn’t that difficult. If United wanted to really go down the anti-fraud road, they would simply run all traffic through a geo-iIP service provider like Neustar or Digital Element and they would identify source, network, A/S number and more about all inbound IP traffic. Assuming United wants to do this, of course.