Looks like my compatibility problems are solved: in the radical way, by completely breaking Setup on all three applications. Setup now crashes on clicking 'Next' at the welcome screen. No Program Compatibility settings work.
These tools are essential for my work. If they don't work I cannot upgrade.
UPDATE 2006-08-31: It appears that whatever was causing this may have been a temporary glitch; Visual Studio 6.0 is now installing. Still, given my experiences with eVC before, I can perhaps be forgiven for jumping the gun?
I can't go in and add a note to the bugs I filed, because as a Customer Public Preview user, while I can submit bug reports using the Beta Feedback tool, I can't log on to Connect to make changes or additional comments. This means I'm wasting someone's time to triage bugs that I now cannot repro.