It just struck me that I could simplify the circuit to remove D(n)b and D(n)c diodes now that only two of the transistors are being activated by the µP directly though; I should now be able to simply connect PA1 to the emitter of Tr1a, and PA0 to the emitter of Tr1b, which wasn't possible with the previous schematic. And of course, follow the same procedure for the others.
In fact, that should remove the need for any of the diodes... eureka
it appears hero's modification to my original idea has made my ground return much simpler. I'll post a new schematic shortly.