Okay, with your help and a bit of research I've discovered a few things (and edited my post accordingly.) Namely, there was a person named Marcus Musurus, who wrote (or compiled?) scholia on Athenaeus. I took another look at Kaibel's text of Athenaeus and found the following footnotes: "ἀλλά τε A: corr. Mus" and "ἐν Mus: μέν A". A bit more research suggests that "Marc." and "A" both refer to the Codex Marcianus. So verb. definitely means "corrected to", and that's both Marc. and Musurus figured out. Still a few mysteries left, which I am currently trying to figure out.