Sounds fairly solid. Really, the little crap your going to run into can't all be accounted for. I know you'll probably need a few trips to radio shack in the midst of doing things, as you might have forgotten about a quick-disconnect here, ran out of 16 gauge heat-shrink tubing there, etc...
Only thing right off is your going to need to splice in parallel the PSU switch that goes to the MB, and to the momentary switch (power button) if I understand correctly this aspect. Which is smart of course, as you don't wish to be stuck solely at the mercy of the M4.
In my experience...It's good to have both + and - fused if you're willing to put the extra effort into it. Frankly, 99% of the time, car fires are caused by some short, usually due to some after-market work done with crimp-kits and electrical tape. If you have the ground fused as well this will prevent this and damage to devices if you put the in-line fuses close enough to the source.
And of course use soldered connectors, connections, heat-shrink tubing, etc..
Doing this causes extra time, cost, and the like, but will save headache, heartache, and you can buy marshmellows in preparation for roasting over the other guy's car with the "professional" install going sour in a few years.
Every time I pay someone else to do stereo grunt work and the like I just end up having to do it over anyhow as they can't even zip-tie the damn crappy electrical-taped ends. knobs. /rant
Best of luck!