Actually, I am not looking for any compensation as I doubt that I could get my wife back on a United flight for a long time to come.

The basic problem was that she moved to a empty row (10 hour flight) on the other side of the plane prior to departure. It was a light load and there were many open seats and rows. Many rows with just one person in them.
For some reason the middle seat between us (both 1P) was assigned despite there being aisle and window seats everywhere. This passenger was not inclined to move and it would be a long flight with all of us together when it was not needed.

I stayed in my original seat so she could have the empty seats to herself.
The doors closed and the mature FA immediately came up to her during taxi and said she could not stay there. She asked why as there were many empty seats and many with only one person in a row. She became angry and said that she had "only paid for one seat". It escalated quickly and my wife was quickly told that she must obey all orders of the flight crew and not talk back. She had only asked why she was singled out and not the others who were moving to empty row. The usual security threats were made. The purser (much younger) came and very loudly reprimanded my wife including finger in face. All of this occurred during taxi and after take off.
The mature FA returned and continued to lecture her. The tone was hostile and threatening and she tried to smile to see if she could try to defuse it somehow. The FA told her to "wipe that smirk off of your face..you need to have your attitude changed by someone". As soon as the FA left she returned to her original seat and stayed there the whole time.
We have no idea what set this off or why they escalated it into a security incident.
Later in the flight, a FA from up front with whom we had flown several times this year came back and told us that the purser and 2 older FA's in the back were telling the whole crew that there was a problem passenger and she came back to see who it was. She was amazed to see it was my wife and quite upset about the obvious false story.
I have no idea why they would do something like this...but there it is. Perhaps they are just unhappy mean old ladies. They tried to get us denied onward flight..so I would guess that they would try to make good on the other threats as well.
Did I mention that my wife is 65 years old and not a threat to anyone.
United has lost some pretty good customers over this.