So likely the LED light pack was functional. Doing the diagnostic before throwing parts at the problem like a shade tree mechanic would is why you are paying for a certified mechanic. Aside from the cracked lens, my opinion is the diagnostic and LED light pack is at their expense. Be sure to keep the replaced LED and BCM, have them second party tested. If they are functional, offer them the opportunity to make the cost right before you sue them. How much labor did they charge for the tail light and lens replacements?