Note that the new map wizard isn't designed to handle fractional map units (even though the program itself does), so trying to input those values would result in a map 3 by 2 units. You'll need to create it as 4 by 3 units and manually resize if you need it that exact or just accept the little bit of extra.

At that scale, you may also consider mapping in feet and not miles, but that does include a bit more of setup when starting with an overland template, so I won't recommend doing that until you get a bit more familiar

Great idea. Sounds fun.

It improves performance of the final conversion step after export. Convenient, but not important as it doesn't change any functionality. The older ImageMagick 6.9 version should have all the files.

Yea, that's it. Portable should be fine. Depending on which ImageMagick version you get (dynamically linked, statically linked, etc, etc,..), there may be some .dll files in the archive, just throw all .dll's in there with convert.exe to be on the safe side. Leave the existing ImageMagick folder as it is.