You are absolutely right on that point - the US would not hesitate to step in when required. Saudi would fall apart in hours without American military assistance.My understanding is that there is a very complex command structure - different branches of the military are controlled by different Princes - an added problem is that those from the old Hashemite families are not permitted high rank. A further problem is that there are conflicts between princes whose military rank may be lower than a commander, can override the orders. Any really severe conflict would be an absolute shambles - note what is happening in Yemen.