In case anyone else finds themselves here needing to make mojitos for a large group, I find Dave Arnold's technique of blender muddling (see his book Liquid Intelligence) to work really well. The prep has a shelf life of 15 minutes, which is ample time to deliver a big round. I dump the leftovers into a large noodle sieve to ready the blender for round 2 and deal with it all in one go at the end of the night.