As everyone knows, I was a big a fan of Kess (I say was, since it seems he's semi-retired now). But I agree with you that Kov gets him outta there at some point. Kessler wasn't that stiff. I mean, he wasn't Kelly Pavlik stiff. Circa 06-07 he had above average athleticism, and he always had above average handspeed. But he wasn't fluid enough to hold off Kov. Kov would force Mikkel into a firefight that he wouldn't win. I don't think he stops Kessler early, because Kess had a pretty good chin, but Kess didn't have a GREAT chin, so I think circa 6-7 Kov gets him outta there.
No way to know at this point. Kessler is better offensively and probably defensively, but getting hit by Kovalev is something he never experienced. Kovalev is bigger, too. I'd lean Kovalev, but if Kessler outpointed him clearly, it wouldn't have been a huge surprise. No reason to believe Kessler's chin wasn't great, either. But, Kovalev's power seems enough that it could take out even great chins.
Kessler was hurt by Froch in both their fights, and dropped by Allen Green. He was also hurt by Calz to the body. He had a solid chin, but a "great" chin is a bit of a stretch. In Mikkel's defense, he'd be the hardest/sharpest puncher Kov has faced to date. I just have a feeling that Kov's pressure and activity would force Kessler into a war, and I don't think he'd come out on the winning end of it.