I should probably start by saying that as of last night, I was able to find effective workarounds to both of my problems. Before I get into that, though...
to get a picture on 5000+ cards you need edid emulation so your edid emulation is working.
This appears to be inaccurate. Right now, I have EDID emulation disabled, and yet a 15khz picture is displaying for both the Windows desktop and Groovymame, while my downscaler is back in storage.
The impression that I got from the Calamity's instructions is that EDID emulation is mostly just a clever way to convince a GPU to use 15khz with the Windows desktop in a way that's flexible. Perhaps, I'm wrong. Regardless, by using ArcadeOSD's "Set as desktop mode" function, I was able to finally get a permanent 640x480 15khz interlaced mode set and working. I hadn't realized that this would remain even after rebooting the computer. There is now no longer a need for EDID emulation as far as I can see, since the PC in question will be connected only the 15khz monitors.
(As an aside, I did try reinstalling CRT Emudriver, and EDID emulation still didn't work.)
The other challenge I had was to somehow get composite sync for my monitors and remove the Extron SS 200 from my chain.
Here's a shot of my gaming setup. (Why is the forum software refusing to let me simply link to this image?) There are four monitors in active use, a big matrix switch, and a couple of
other Extron boxes doing other stuff with sync.
It would take forever to explain everything that's really going on in this mess. Suffice to say, I came up with a hackish but nonetheless effective solution to get one of my other Extrons to combine sync from this PC and output it to all of the monitors, and still have everything be controlled by buttons and switches - no unlplugging of cables or anything necessary. Thank god the Extron 202 Plus has two inputs!
If anyone is curious, the problem with the Extron SS 200 is that it doesn't deal with certain frequencies well. It's OK with some forms of 15khz, but some combinations of H and V sync make it output a goofy C sync that messes up the picture. Changing the settings on the device does not help, either.
While I'd still prefer it if my Groovymame PC could output composite sync, I can live with the current situation. Thank you MrMikeZH for taking the time to try to help me. While I would still be curious if anyone has any ideas about enabling composite sync, I'll consider this "case closed" for now.
Thanks again.