Then that's a situation you need to address with WDW.
I have a disabled tag due to very bad asthma, I seldom use it--especially not at the mall, the gorcery store, etc. But if DH and I go to WDW on a hot summer day, we'd use it. If I have an asthma attack, being out in the heat and sun for the 15 minutes it would take DH to get the car could mean needed to be taken by amblance to the hospital as opposed to going home.
While I dont have a mobility issue, it doesn't mean that my condition is any less worthy of a handicapped parking spot than anyone elses. Why would gram in her wheelchair be any less worthy of the spot than you? Because what you are asking is basically for her to have to wheel up from the back as well.
Anne