The distances involved in a trip to WDW are surprising to most people. Just to go around WS is a little over a mile and I would estimate that it's probably almost that far to get from the park turnstiles to entrance area to WS. So, if you haven't already thought of it, you probably would want to consider renting a wheelchair for the 91 yr old. You can get info about doing that by following the link in my signature to the disABILITIES Board.
The walk from the YC/BC to the Epcot area is not that far, but it is uphill toward Epcot and would be a long way for a 91 year old to walk. BC is closer to Epcot, but YC is closer to the docks.
This link to a map of YC/BC should help.
From the lobby at YC, it's a straight shot out to the docks. DH and I were just at BCV in July and the docks listed on the map were the only ones we saw. The ones listed on the map as the Transportation dock by the Lighthouse are where the shuttle boats leave and dock. I'm not sure, but I think the Illumination Cruises probably sail from the ones that are listed on the map as the Marina.
I can think of 3 alternatives:
1) Make your visit to Epcot to eat, then all leave Epcot, drop your guests off at YC lobby and have them wait inside the lobby while you park the car. Walk straight thru the lobby to the docks. You can also valet park; it's quite a hike from the parking area to the hotel lobby. Because the parking lot is far from the lobby, valet parking is free for people with wheelchairs or handicapped parking stickers.
After Illuminations, reverse the process to get back to your car.
2) Have most of your party walk or take the boat to YC while the car driver transfers the car to YC. Meet at the dock and go on your cruise. Then afterwards, most of the party can wait outside the lobby while the driver pulls the car up.
3) Park at Epcot, take the boat from Epcot and back after your Illuminations Cruise. The thing I don't know about this is whether the baots leaving YC to go back to Epcot will take passengers there after the park is closed. This might put more stress on the 91 yr old to hurry because everyone would be worried about getting back to the parking lot.
whoops, link repaired!