No, it's still Chungha (29) 64%
Yes, Sejeong is the most successful post-IOI member (14) 31%
Neither Chungha nor Sejeong are the most successful post-IOI members (2) 4%
Sejeong was in The Uncanny Counter, which was a pretty popular drama both in Korea(average rating of 7%) and overseas. She was also on Busted!(a variety show with 3 seasons) for a few episodes along with Sehun, Yoo Jaesuk etc. Arguably, we would need to wait for her future acting endavors, but she's doing pretty well so far.