It wasn't a mistake to sign McCutchen, it was a mistake to play him so much. Compared to the results from other guys they could have signed, McCutchen did fine.
It wasn't a mistake to rely on Ethan Small, because they didn't. He was stashed in Nashville as the potential 7th reliever. They had a couple of AAAA SP who could fill in at need. It wasn't a mistake bringing up Jason Alexander, it was a mistake to not have an alternative, which forced him to start 11 games, when AAAA really shouldn't get more than 4 starts. At or before the trade deadline, they should have acquired some mediocre, old SP to fill in the rotation--they foresaw Lauer, Houser, and Peralta returning, but some of them returned later than they hoped, and were on short pitch counts.
The deadline deals didn't work out in retrospect, but I'm guessing they didn't have any great opportunities they passed on. If the Hader deal ruined the clubhouse, that's on the players. They're getting paid lots of money to play a game and they shouldn't let that affect them.