First, welcome to the forums It looks like you've done a good amount of research before posting which is excellent. To answer your questions:
1A) Yes, you should get a distribution block, fuse the 4GA wire for 100A, use 8GA wire to go between the amp and the distribution block fused at 60A, and 12GA fused at something like 20A (check the rating for the m4) to go to the carpc psu.
1C) You may have to get a 12v powered USB hub or two.
1D) If your screen does not have a built in regulator, you should power it off the PSU. That's as easy as running a 12v wire from the carpc to the screen.
1E) Inverters are never better to power a carpc, but if you buy one so you can plug in other stuff, sure. Just do NOT run the carpc off of it. If you need USB hubs get a 12v powered one.
2A) It should work around the world
2B)All of those should be fine as long as they have clear view of the sky (or source of the 802.11/bluetooth)
3) Personally I've had the Xenarc 700IDT for two years now, it works wonderfully. I've only had to send it back once, and that was because I broke it taking it apart (which they fixed under warantee). It's not the brightest screen, but it works great.
4) As I said, it sounds like you've done your research. For your USB hubs you may need to look into a 5v regulator like the Carnetix(sp). For OBD2, I suggest the obdpros.com adapter. I've seen problems on the forum with people getting the ElmScan to work with your standard ELM compatible software.
Some people here may disagree with me, and i could be wrong on some of these points, so of course do more research before taking anything I say at face value. These are just things I've learned as I've done this stuff over the past 5 years.