I know the basic purpose is to fix the muzzle device so that it is not removable.  It seems that most people pin it by drilling a hole that is off center, through the edge of the threads, and out the other side.  But, is it legal to drill or mill a hole dead center so that it drops straight down and kisses the threads?  This way the end of the pin would lock into the threads and not allow the muzzle device to turn.

yes but you better go a little further than just kissing the threads.  

you might also cut your pin a little short and tap it in just under flush and tac weld the pin in place.  

Most people drill it 'on center' and slightly dealper then the bottom of the threads. You also HAVE to weld over it so the pin is not accessible. Otherwise you will not be complying with BATF regulations.

is one pin acceptable ? or do you need two(or more) to be legal ?