(Since I can't post a reply without registering, I'm starting another thread.)
rewboss, thank you for your response. I was trained as a copyeditor by a curmudgeonly septuagenarian journalist. Would you find it redundant to say, "He did this, and now he does that"? Or "He did this, and later he did that"? I don't find the "and" redundant in any of these cases. "He did this, and he did that" is a reasonable sentence. Why should adding "then" as a modifier to the second part make the "and" unnecessary?
Again, I'd be very interested in a citation from a grammar text or style guide on this point.
Good question. I'm struggling here, I'll be honest.
My best guess is that "and" emphasises a contrast, but with "then" you're not contrasting anything.