GianfrancoPa:
I had a very hard experience with 5940mux library, for this reason I tried to
improve the basic TLC5940 library.
I greatly appreciate the reply, but if I understand correctly, the library and setup from Grumpy Mike which I'm trying to get working is different from the Tlc5940Mux library and setup from Alex Leone. It seems like I'm really close to getting this working so I'd rather try to figure this out before trying a whole different setup. Besides, his setup seems less documented so I think my chances of getting it working would be smaller.
As for additional troubleshooting:
I tried looking at the serial monitor to see if there was any obvious problems there, but nothing displayed. I did set the speed to 57600.
I've checked the hardware over and over, but can't figure out what/if I screwed up. I still just get solid rows of unique colors, but not individual LEDs. I'm still hunting for FETs more like what Grumpy Mike used, but am still unclear if the FETs I'm using now are definitely a problem.