I know its too late for you, but maybe this will help someone else.
When the funeral home sends in the request for honors, the director has to specifically request the three volley salute. Once he sends that in, they'll receive a confirmation that will include number of personnel on the detail and what will be performed at the funerals. As you stated, sometimes, for whatever resson, the personnel are just not available and the rifle volleys cant be supported.
However, there are many organizations that will work with an existing Detail to provide the volleys if the three volley salute isn't on the menu. Either the Director or the family can contact the VFW and get contact info of these organizations. Some are private organizations, but most Ive ever worked with were from Natl Guard Armories around the state.
I realize its a difficult time for everyone, it just takes some communication and a small bit of extra effort, but it can be done.