Well I did try all the various combinations outlined above and none of them kept the audio in sync, unfortunately.
So I went back to the approach which synced audio correctly: PS3 -> (hdmi) -> TV -> (optical out) -> Denon
After a bit more fussing with the audio settings on the PS3 I eventually got the THX sound to come through as well
I think the key was reconfiguring the supported audio types on the PS3 (which turned out to be just PCM for the communication with the TV), and then setting the HDMI audio out to be PCM.
JDSmoothie, your advice helped me understand this stuff enough to make more targeted changes, so thanks again for the help!