You have an advanced architecture which is not common in this forum. Those who tried to answer are all regulars with experience in everything Moodle including large installations. If they can't answer your questions, you have dwindling chances here. Try external resources, like Moodle Partners as Marcus proposed or try to contactA new "Large scale Moodle deployment" support group . Before you ask, No, I'm not in that. So you have to ask them.