Of the three players you name, only one is really outstanding, Stewart. Sellers is a mediocre QB and Harbor is a great athlete learning a position.
Everyone was so excited that Harbor signed because he was a 5 star recruit. No one really stopped to think about the reason he signed with SC over the other more successful teams recruiting him. It was because Beamer was the only coach who agreed to let him play WR and run track in college while playing football. He had never played WR before and every other coach told him they wouldn't let him switch to WR or run track.
Sellers is a similar story. Sellers was a great HS QB, but he wasn't really a pure QB. He was a great athlete/RB playing the QB position. That is a great plan for a HS QB, not so much in college. He is a great athlete, but not an SEC level QB. When people talk about a QB and say "he is great but his passing needs work", they are saying "he isn't great".
I think Beamer gets his 6 wins in the regular season and is safe for a couple more years. Unfortunately, I think Beamer is a 5-7 regular season wins coach, regardless of how great the fans think his coordinator and asst. coach hires are. Donati has to decide if that is acceptable. Everybody "likes Shane", the question is, do they like him enough to be happy with a fairly low ceiling?