The epic-length post is like those movies that would've been 10 minutes if the people just talked with each other.
In this case, if the artist had just told the agent what they wanted, agent would've gotten to do their job, and artist would've avoided years of turmoil.
(I'm sure a real agent could negotiate much better than I could; I'm only suggesting that an agent could straighten this out and maybe get better terms, without threatening the deal.)
If you're talking directly to someone about buying their house and you go, "your house is a piece of shit, everything is broken, knock $XX,000 off or I walk", they're going to tell you to go fuck yourself. Once you filter it through two agents they hear "well, the buyer is concerned about X, which is very reasonable, but Y is just being picky, we should come back to them with an offer for $ZZ0,000 and I think that's very fair." Sure, they're not happy about selling their house for less, and maybe no agreement can be reached, but the agents take a lot of the emotion and confrontation out of haggling so that you don't end up hating the other party before you can reach an agreement.
So it goes with all negotiations.
I was thinking this throughout a large portion of the article. Imagine how different things would have been if Julian and Carl stopped emailing each other "rapidly" and "frantically" and picked up the phone (or setup a Zoom meeting or whatever).
Your point that OP would have been way better of if they did let their agent handle it from the beginning was made quite clear already:
> Looking back, I can see that I should, at this point, have handed the conversation over to my agent, Charlie, who negotiates my book deals with publishers, and who is a complete darling, and good at this sort of stuff. But I got stuck. I think I thought that bringing in my agent would potentially mess up this pure art relationship. Maybe he would be too ruthless, and it would all fall apart! Of course, now I can see that it would have had the opposite effect; Markus and I could have talked art, and Carl and Charlie could have talked money, and everyone would have been much happier.