According to the author of a 2016interview with Miller, the White House sent over this afterthought requesting it be attributed to Miller:
“It is the single greatest honor of my life to work for President Trump and to support his incredible agenda.”
I also don't believe Barr chose these words:
No tactic, no matter how abusive and deceitful, was out of bounds.... The nadir of this campaign was the effort to cripple, if not oust, your administration with frenzied and baseless accusations of collusion with Russia.”
The New Yorker article describes Miller as being "...convinced that a cabal of deep-state actors was trying to thwart Trump’s agenda."
Of course, I have no idea who wrote Barr's letter. But I'd bet the scenario went like this: Trump tweets his criticism about Barr for not playing footsie on election fraud. Trump stews over the weekend and commands Barr's presence on Monday morning. Trump fires him, thinking he's being generous by suggesting he submit a resignation, which of course has already been drafted. Trump presents the letter to Barr. Barr either signs on the spot or crosses out a few words but the changes are quickly made. Barr decides not to make a fuss.
I think Barr is just looking out for his next meal and wants it to include a rich dessert. He fears without Trump's blessing, his next job will be the equivalent of hamburgers and a Dairy Queen.
Barr was never our Attorney-General. He was Donald Trump's personal lawyer. He replaced Jefferson Sessions, so of course he knew his role and his place when he took the job. His job was not to serve America or justice, but to please and praise the man with a desk in the oval office, Donald Trump.
At the end, he realized Trump and Rudy's fraud claims were going nowhere, Trump would be out on January 20, and he'd be out of a job. So why back a losing proposition? Trump thinks he fired Barr when he told him to "go now", rather than in a month. But Barr probably had his new job all sewn up and it's likely a job Trump could jeopardize. By agreeing to go now, at least he'll get to keep it. He'll also get a few weeks off and avoid being associated with the Walk of Shame -- the image of Trump, head down, as he takes his final walk across the cold, wet White House lawn with Melania, her six inch Stilettos sinking into the mud, to a waiting helicopter for his final exit from Washington-- all the while vowing he'll be back.
Trump thought "Go Now" would be a shaming punishment for Barr. I think it will turn out to be the opposite. By playing fanboy as he walked out the door, Barr may even get the last laugh. After all, who knows better than Barr what other investigations are going on at the Department of Justice that touch upon Trump, Rudy or other insiders? Federal grand juries may or may not be meeting due to Coronavirus (they are on hold in the District of Colorado) but you can bet the AUSA's, FBI, IRS and other law enforcement agencies are talking and reviewing documents daily.
It make take a while, but at some point Donald Trump will run out of favors and have to face the music and pay his bill, which I think will be big. While William Barr will be remembered as the AG who refused to go along with Trump and Rudy's phony election fraud claims and undermine the rule of law. With Republicans, anything is possible, even turning Barr into a hero who saved our democracy.