] That's the guilt that she is ready to admit, and she has all but admitted it, and then Hamlet starts speaking to the air.

Yes, and she's obviously been harboring some sort of guilt, though very sublimated, from the beginning - her complaints to Hamlet about his moping and wearing of black smack of the alcoholic who's unhappy that others won't drink with him, the partier who reviles the party-pooper. If it were purely concern for him it would be more of the Darling-are-you-okay,-can-I-do-anything-to-make-you-feel-better? variety, rather than the What's-the-matter-with-you,-quitcher-moping-yer-bringing-us-all-down variety it is.