From the last episode (Season 4, Episode 76) we know what Eri's quirk is. Rewinding. If she is able te rewind a something to a earlier time, wouldn't she be able to restore Lemillion's body to the way he was before he got shot by the bullet from Chiasaki? This way Lemillio should be able to regain his quirk?
P.S. i dont think this would work on All Might because he passed his quirk (One For All) on to Deku.