If you go to post #6 of the disABILITIES FAQs thread, there is information about GACs. It was written about Disney World and I noticed you are going to Disney Land, but the information about how GACs work is similar.
GACs are not meant to shorten or eliminate waits in line. Sometimes using a GAC will even make your wait longer, but the wait will not be in the usual line.
Many attractions, especially at MK have boarding on one side of a ride track or waterway and the exit is one the other side. In order to make sure that mobility devices are on the same side as the guest when they exit, people with wheelchairs and
ECVs board at the exit (usually after waiting in the regular line). Sometimes people with special needs who do not have wheelchairs also board at the exit because it means they can wait in a place with less people.
Small World at WDW is an example of a line that works like this.
This may sound good, but the handicapped line is often backed up because of the number of people with special needs who are waiting to board. We have been in the handicapped line waiting to board Small World for 25 minutes at a time when the 'regular' line was posted as a 5 minute wait and some boats were being sent out not totally filled because people were not walking up fast enough.
If any of your clients are using wheelchairs, they will probably have to use these lines because the 'regular' line will not be accessible in
Disneyland. The lines at Disney's California Adventure are wheelchair accessible because the park was built after the ADA was enacted.
Because Disneyland is an old park, each attraction is going to work a little different. I would expect that there will not be places to sit unless your clients have wheelchairs or walkers with seats with them.
At least at WDW, the alternate waiting areas for shows are a part of the preshow area that has been separated off from the rest of the room with a chain or rope. It will usually be in sight of the 'regular' line, but separated a bit from it.
So, my advice would be to look at the regular line and decide at each attraction if that would be a better option for your group. Hopefully, someone with recent Disneyland experience will speak up and help you out.