I agree with you 100%. and I also agree that Karlsson will not be suspended on this play. It was a clean hit, imho, but due to a number of factors Nathan's head did take a lot of impact.
This is purely conjecture on my part, but I also noticed Nathan seemed to lose his composure a bit after that hit, and at least twice stepped up for a big hit which led to odd man rushes. He seemed to let his emotions get the better of him. Shortly thereafter, he rode the pines. MT has sat Subban in the past for similar things, which leads me to believe it was more disciplinary. Again, I'm just guessing here.
Also, I don't pretend to know concussion protocol, but I don't imagine if you go out with one that you stay on the bench for the next hour.
Bottom line, I think if Beaulieu is healthy I believe he plays. He's just too good (when he's focused and disciplined) to sit on the bench or be the press box. This isn't a knock against Gonchar, I just think Nathan is a better D man at this point of the season.