I used AppCleaner again and this time removed both MC25 and MC26. I then searched my hard drive for anything containing media center or jriver and pruned a bunch of files. After re-installing 26.0.18 now, it appears to have actually reset (new install key, skins changed to default, no library, etc).
I was playing a stream for about 20 minutes, no problem. Now I'm back to getting a beachball if I command+tab to MC, Activity Monitor shows Not Responding - though audio is still playing. If I click on the MC icon in my dock, nothing happens. If I right click, the option I see is "Force Quit."
Edit: MC stayed on Not Responding for about 5 minutes while I typed this. It just popped up and now I can see in Activity Monitor it's using 30%+ CPU. All I'm playing is a 22khz MP3 stream. So maybe it was just building thumbnails, etc from a new install... I'll let it run wild for a while.