The only reason Betts was in his walk season was because the Red Sox decided to play broke. The Dodgers signed up to an extension before entering FA. So from the perspective of Red Sox fans they aren't looking at it as a walk year trade.
It's not about not trusting your starter in the first. It's about not trusting him to face the top of the lineup a third time through. You use the opener so he can face the bottom of the lineup a third time through, and not the top.
More than just the PR, he's also a locker room cancer. And he's not a CY Young level pitcher anymore, his talent is nowhere near enough to justify the headache.
Move down in the lineup, no. When he wants to give Riley a day off it will be against a lefty, but he's not going to platooned against lefties the way Carpenter is.