I mostly agree with what you're saying, but I'd add that I haven't seen any teams actually run at him since he was a freshman. I haven't seen any O-lineman handle him 1-on-1. I absolutely agree that Chamberlain has historically underperformed with lots of coaching turnover, but they have had some decent coaches recently, though I can't speak to his D-line position coach specifically. Part of why he hasn't dominated teams is because he plays in the middle, and he has so little support around him. If you want to beat Chamberlain, have your 3 interior linemen block Hutmacher, and then run or pass to the outside: there isn't much there. One other thing with the coaching is that he's been at a small school while playing both ways, so there just isn't time to go into the level of technique that he'd have at any school with a full-time D-line coach where all he's working on is D-line technique. I'm not aware of him attending a lot of summer camps either because he's always wrestling, and you basically have a human wrecking machine who has not really had the time or opportunity to learn much technique. It makes it harder to evaluate him in that he's pretty much always a 2-gap player, meaning he's supposed to stand up the middle of the line (at least 2, often 3 blockers), so he doesn't really have either the training or the opportunity to show the ability to get much penetration.
I very much disagree about him having slow feet. I'm not sure what you were watching, but he has phenomenal/athletic feet for a lineman, let alone a 300+ lb lineman.
I'm hopeful that Chamberlain will be good enough to make the playoffs this year. I'd love to see them make it to the second round with a low seed so someone like Sioux Falls Christian or Thomas More--power football teams who haven't played him--would have to decide how to block him, and whether or not they could run at him. I'd say that it's a reasonable possibility that that happens, depending on who they'd draw for that 1st round game.