Finally got everything working! The final problem was 100% my inability to read. As Orgia suggested, and catskull alluded to, my "link cable" was the final issue. I for some reason was using pin 19 instead of 16, so fixing this mistake got it talking immediately.

This doesn't account for my initial problem (and rich's), so I'll have to continue investigating if I want to use trash's most recent builds, but I'm happy to have it making noise in the meantime. If I find out anything more that may be useful to anyone else with similar issues, I'll follow up here.

Thanks again to everybody who offered help and advice - I'll post the finished build in another post when I'm done!