The abbreviated plan looks good to me.

If you use S/PDIF w/ transformer isolation or toslink optical there should be no issues with alternator power corrupting the digital signal -- but you may need to be careful powering the DAC. For example, I wouldn't be surprised to see problems powering the DAC with USB power from the carputer, because the potentials of the grounds of the amp and carputer could be "bouncing" relative to one another. On the other hand I wouldn't be surprised to see things work either.

I posted a DIY version of a signal processor that uses an IC from the family likely used by the nanoDSP (ADAU1442 / 1445 / 1446, see post here), where the DIY version has the benefit of affording access to the full potential of the chip with analog device's SigmaStudio software. It requires circuit soldering, however, which can be tedious -- and I think the DIY version would also require purchase of something like a EVAL-ADUSB2EBZ "USBi" to communicate with the IC, which costs about as much as the nano.

Also, if you're into mods, I'd suggest putting the DACs in the amps, thereby bypassing the amps' gains & cross-overs, etc, since you'll be using your own crossover ahead of the amp.

Lastly, what about nixing the nano altogether and doing the signal processing with the carputer? There's a thread related to that process (which uses a different miniDSP device to accommodate multiple digital channels) here.