POR has a nice boat that goes to DD. If your DD likes boat rides, you can also rent boats at the marina and take a trip toward OKW and DD. One nice thing is that the water channel is fairly narrow (which might be more comfortable for a child with autism than going on the big lake at the MK resorts).
One thing to be aware of for going to the MK from any resorts (those not on the monorail, at least), is that if you take the bus, you will be delivered very close to the turnstiles to get into the park. If you drive, you will need to take the parking lot tram to the TTC and then take a boat or monorail to MK. If you have a handicapped parking permit and park in that area, you still need to go to the TTC and the walk can be quite far.
I don't know about the bedrail on the trundle bed question either, but I know someone who came up with a bedrail substitute that worked for their child and was packable. They took a pool noodle and a sheet. Along one side of the sheet, they sewed a channel just big enough for the pool noodle to slide in. They used a knife to cut the pool noodle into small enough pieces to fit into their suitcase. When they got to the resort, they put the noodle into the channel on the sheet, put that along the outside edge of the bed (about 6 inches from the edge) and then tucked the other side of the sheet under the mattress on the opposite side. The weight of the child kept the sheet in place and the pool noodle was enough to keep him from rolling out of bed.
There are also some combination inflatable mattress/sleeping bag things that I saw in a
Walmart ad that might do the trick for you. I have not seen them in person, but they look really cute.
They have
Dora the Explorer and also
Disney Princess ones. That way, if you didn't get a trundle bed, she would still have her own little space.