You and I have read different versions about what was said to Devers. But no matter.
I think we can agree that playing 3b was a big deal for Devers, who had played 3b for 8 straight years. Also that hiring Bregman, a good hitter and a gold glove third baseman, meant that Devers had to leave 3b. Because Casas was at 1b, Devers had to be DH.
When Casas, who was having a bad season, injured himself running to 1b (last year his injury came from swinging a bat), Breslow saw the perfect opportunity to move Devers to 1b and use Yoshida or whoever at DH. At the time, I thought leaving Devers as DH got the best use out of him. As it turns out, Toro/Gonzalez have actually been better at 1b than Casas was.
However, once the trade with the Giants was done, I was fine with it, partly because of your point that paying Devers $30M to be DH and paying Yoshida $18M not to DH didn't make sense. I also noted that the 6 game winning streak occurred because of the Sox pitching and not because of anything Devers did. I also said--after Devers was traded--that Devers became trade material the day Bregman was announced as the Sox third baseman.
So, while I do like to defend Devers if only because he was an excellent DH, I have always been fine with his departure. Whether or not he was lied to loses its significance as each day goes by.