personally i dont mind that mirio got his quirk back - it was basically promised to him, no? at least its been hinted, its been said that they will see and try to fix his quirk. what i do have a problem with is the timing - its too much everything at once. but back to mirio thing. i also believe that eri herself wanted to fix mirio since he was the one to save her. she, a little girl, who was ready to die so others wouldnt be hurt. wouldnt she want to, in a way, thank him for showing her genuine kindness and care?
BUT ALSO we dont actually know if it was eri, there is an antidote for the quirk removal bullet, maybe they somehow got a hold of it or maybe they somehow made their own antidote based on the bullet they got when tamaki got shot with it (im talking abt the one which they tested and found human cells in)