Ok, no answer yet.
Anyway, i decided to have the 4 sided allen key made from the shaft of an old shock absorber i had lying around. Using my vernier caliper, i had the allen key made measuring 13.7mm (on all sides of course). I assume 13.7 is approximate since my caliper must be somewhat worn by now. I`ve only actually tried it on the rear diff fill/drain plugs, and it fits nice and snug. I havent tried it on the front diff as i still have to remove the bottom plate??, but i assume (hope) the plugs are the same size. I then tried it on the transfer case plug (dont know if its the fill or drain plug), but it did not fit! I measured it and its approximately 13.7mm.
So anyone have answers for my questions #1, #2, and #3?