I'll concede that your first statement is mostly accurate; looking back, most people on this forum did caveat things with "if true" or something to that effect.
Since we're speculating, to your second point, predictability is the is one of the reasons I'm very skeptical about the allegations contained in this latest book. Most 3-4 star generals are politicians or at least very politically savvy once they get to that level, it's how they got there in the first place. In my experience, all the generals I've been around or worked with at that level invoke CYA in almost everything they do and say (I think they take a class in it prior to pinning on that third star). Slight derail, as I understand it (I've never done a puzzle palace tour thankfully) it used to be the JCS was actually one leadership level where this wasn't always true, since once you've made JCS or CJCS you've hit the ceiling, there's no where else to go. My buddies up in the Pentagon have told me that dynamic changed appreciably once Mattis was made SECDEF; several generals now had the attitude that they were still potentially upwardly mobile and started acting as such, but I digress.
I find it very difficult to believe that Milley told the Chinese he'd give them a heads up before the US attacked or that he made other officers swear an oath to him regarding the use of nuclear weapons because if he did those things too many people are/were in the know and as has been stated on this forum, those are illegal acts that would result in UCMJ actions. If the people in the room for those calls or the officers he allegedly made swear an oath get hauled in front of Congress, the IG, or a courts martial, they're not going to fall on their swords for the good general. He'd be proper fucked and he's smart enough to know that.
Another thing to consider, at least regarding the timing of the first call. What if Trump had won the election? I find it very difficult (though not impossible, as I said I'll be interested in what he says to Congress on the 28th) to believe that Milley would take such actions or say such things knowing the guy he allegedly is conspiring against might still be his boss. Trump sure as shit wasn't going to give him a pardon or be forgiving about it. To be clear, I'm not arguing that Milley didn't do and say these things because of his honorable character, I'm saying I think it's very unlikely because he's a political creature.
On the Chris Miller denial point in particular, we'll have to agree to disagree. In a follow up to his statement to Fox about the phone calls, he told Politico, despite his pretty forceful rebuke:
"I imagine there was a perfunctory exchange between us and our staffs about coordinating phone calls and messages for the day.”
“I don’t recall the specifics, and it certainly wasn’t in a detailed or more formal way," he added. "It was more perfunctory/routine.”
Always have an out/CYA.
Also, the guy was in the seat as SECDEF for 53 days; before that he was ASD/SOLIC for only 3 months. In most staff gigs it takes a min 4-6 weeks to get your head wrapped around the day-in/out basics. At the time of the second phone call, he was two days removed from the storming of the capital debacle and he also was working to catch up on the transition to the Biden administration, which had been delayed due to Trump's refusal to concede the election results. My guess (once again I'm speculating) is that his office had been advised of the call and that he had other things going on.
FWIW, I think at minimum McKenzie and Milley should resign over how the Afghan withdrawal was conducted, I'm just not fired up over this story unless more corroborating facts come out. My personal take on this right now is that it smells of the 10% truth rule on Friday night stories at the squadron bar.