I don't understand how you can say this about Merrill while also saying the Pirates "mishandled" Davis, the two teams did almost identical things with Merrill and Davis. The Pirates had a surplus at catcher with Endy looking like the future at catcher, so they gradually started to move Davis from C to RF to start the 2023 season. By the time he was called up, Davis had played about 130 innings at RF in the 2023 season between Altoona and Indy. Merrill only got spring training with the Padres to learn how to play CF, he got 76 innings in spring training to shift from SS to CF and was immediately named their starting CF to open the 2024 season.
The only way the Padres moving Merrill to CF was any different was because Merrill actually panned out doing it, while Davis didn't. Maybe you can argue that moving from SS to CF is easier than C to RF, but I really don't see how the Pirates did anything differently than the Padres there. Davis just didn't pan out because he seems mentally fragile, and ironically he actually hit better as a RF in 2023 than he hit as a C in 2024.
From watching Davis in the MLB, the impression I get is that his problems are mental and not really a problem of the Pirates development. The dude has been killing it in the minors, he just has some sort of mental block that is preventing him from putting it together in the MLB.
But back to the Endy talk, it does seem that their thought process was basically "now that you made the team as a C, we'll look at you as a 1B option as well". Which you can reasonably argue shows bad planning on their end, because they've been trying to push Frazier and Triolo to 1B in the spring and could have spent more time doing that with Endy even if he was AAA bound. Especially with Davis in AAA, you could still use Endy at 1B now and then. That said, they were put in this mess with trying to push Endy to 1B because Cherington didn't bother giving the team actual 1B options beyond Horwitz.
I don't understand why they brought back Frazier instead of keeping Joe. Joe offers the same OF flexibility as Frazier, he complements Horwitz well as a platoon option (even with his mediocre overall bat, he's good against lefties) and the team doesn't really need another 2B bench piece with both Triolo and Bae as bench pieces.