The ID of the move the enemy monster(s) picked is in the variable mv, which is defined at that point... so just change a_comm to mv and it should work as you intended. Speed is only checked when actions are slated / enqueued, they're sorted in a priority queue using the speed as priority and executed in that order.
It might be cleaner to add your speed variable to the case 30 block a bit further down, since that's where slated actions get finalized. There's a block here that lowers effective speed by -75% if the monster is paralyzed, and you could add in a new block that checks if the action type (battleaction_COMM_TYPE element of the array) is an attack, and if so, multiplies the SPEEDPRIO element with the move's speed. It shouldn't matter where you put this as long as it's before the "Finally, enqueue the action" bit, but it's probably safest to put it after the ailment block.