So, I was able to do the mixdown on chapter 2 of "Boats" without using a RAM disk, without glitching. The key seems to be using a different drive to write the mixed track. In other words, asking DSP Quattro to play three different audio files and mix them to an output recorder which writes the result to a file on the same drive will result in glitched audio. I thought that perhaps the problem might be streaming multiple files like that, so I tried putting the files in a RAM disk. This worked, but it seems this is unnecessary. The real performance problem seems to pop up when reading and writing to the same drive.
I'm not sure if it is because the 80 GiB internal drive in the Mac Mini is slow, or if the same problem would likely occur with my regular "benchmark" drives, which are Seagate Barracuda 7200 RPM drives with an 8 MiB cache. (I've purchased several of these in different sizes and they seem extremely reliable and fairly quiet).
It probably has something to do with the ability to safely do read-caching but not write-caching, if that makes sense. It leads me to wonder whether I could make a simple configuration change that would allow writes to the drive to be cached. And then that leads me to wonder whether I would really want to do that, or whether that starts to risk data integrity problems in general, given that the same drive is holding the OS, my applications, my logs, etc.
In any case, it starts to become clear why you see recommendations to use a separate drive for audio files!