Been working at this for awhile, along with great help from Brian at Presonus Tech.
The issue is not the video player, it's the audio codec. When the player says No Codec or it doesn't load/play video, apparently it's not only the lack of a correct video player codec that's causing the problem.
The workaround that I found, after putting maybe 20 hours into this, is quite simple, really: Just split off your video clip into video only. This can be something easy like an H264 MP4 format in HD, for instance. And then split off the same video clip into audio only. I'm on Win7 Pro 64 bit so I split it into a WAV file.
Then load the audio track into a spare guide track and your video into your player, turn the sync link on - and voila!
It's a workaround, and I'll get to the real bottom of the problem, but this works, and it works now, for me!