Hollywood Studios has the smallest handicapped parking lot. We have had problems parking there in the past when we arrived after 10 am.
We just got back from WDW on 10/30. We rode the bus to the Studio; the bus stops sort of border the handicapped parking area, so we could see it was very full when we arrived and when we left.
If the lot is full, your best bet would be to drop her off and then go park elsewhere. That is what we have done when the handicapped lot is full. The drop off point at most of the parks is near the front of the handicapped parking area or you can always stop behind the row of parked cars and leave her off.
I don't know if she will have a wheelchair,
ECV or something else to help with mobility, but if she has a handicapped parking permit, she will need something to help with the distances in the parks. The handicapped parking areas are the closest walk in to the park, but some of them are not exactly close and she may need assistance to get to the park entrance and the area where
ECVs and wheelchairs can be rented.
There are some 'shuttle' wheelchairs in the parking lots that can be used to get from the parking lot to the wheelchair/
ECV rental area. There is no guarantee that there will be any wheelchairs in the parking area when you arrive though. There is no tram service from the handicapped parking areas.
The MK handicapped parking lot is quite a trek to the Ticket and Transportation Center, where you will either need to walk up a steep ramp to take the monorail or walk a bit farther to get to the boat. A lot of people who are staying on site choose to take the bus to MK for that reason.
There is more information about WDW transporation/parking in the disABILITIES FAQs thread. In post #2 of that thread there is information about renting wheelchairs and ECVs. You can find the disABILITIES FAQs thread stuck near the top of this board or you can follow the link in my signature to get there.