I would block a jab/cross combination with the "same" elbow, so if he throws a left jab, i use my left elbow and the cross usually with my right elbow. I also noticed that keeping the lead/back hand setup while using CM is very important also to close the gap between your two arms. In other words: never hold your hands up perfectly symmetrical towards the enemy.
The two things I think are important here would be 1. the movement of the arms "right elbow to left knee, left elbow to right knee" and combing the hair and 2. (the obvious) actually do something about his attack, countering etc. since CM is just a shell for us to rely on but eventually expand from and develop our own attributes and style (that's my view of it).
"Therefore, those forearm movements in Crazy Monkey aren't really BLOCKS, like something equivalant in Karate, but just a way of forming a "shield" around my head while making the gap between my arms hard to catch with punches."
that sounds 100% correct. when i showed kids CM they didnt really grasp the "block with your forearms" but when i told them to hit the others fists with their elbows, they understood the movement.