Bolts are best specified by outside diameter of thread x thread pitch. Thread pitch for metric bolts is how many mm between threads.
I thought it was the depth of the threa rather than the distance between. So for internal threads you can measure the internal hole (including the thread) and then take an educated guess, so if your hole is metric and 8.75mm wide, its going to be an M10x1.25 thread. Useful if you can get a verier caliper or a hole guage in, but not a thread guage. Just measure it accurately.
no, it is the pitch - you would need to look up the ISO drawing or whatever but it so happens, either by coincidence or design, that the minor diameter i.e. the hole you would drill is more or less the same as the major diameter less the thread pitch.