As for Answer #4a: yes, I was using the composite cable from Roku to receiver audio L and R, and that resulted in bad lip synch. But now I'm not using the composite at all because:
Answer#4b: I tried your suggestion to output TV audio to my receiver and the bad lip sync disappeared. You're right: the fact that lip sync was correct when using the TV's HDMI audio was an important clue. So this problem is fixed.