All these things you listed are to be blamed on Tata.
The reason why we allow our opponents to win so many headers on set pieces is because Tata doesn’t inspire confidence in our defense
He is not supposed to inspire confidence, he is a backup player for a key player so he is not supposed to play this much maybe only 10 games every season including the cup
just like Theo and Leao as key players, just like Benzima and Courtois in Real Madrid
if he was better, why would he accept the bench?
against Roma there was nothing to cause trouble to confidence, Tonali left Ibanez in the first goal and ran away from Matic who headed it first and Tata saved it then Abraham scored the second goal while Kalulu was watching
Kalulu started doing weird mistakes before the world cup, he kept on failing to control his ball and giving it to the opponents i think against Cremonise then Fiorentina and he picked up where he left after the break
back up players for key players would always suck, because they don't get much chance to play so Leao's back up would only start 8-10 games in the season
but Giroud or Calabria or Tonai would be involved in rotation so their back up would be in a better shape
signing a very good player in Theo's position or Leao's position or Manian's position would be a waste.