I can't really answer your monorail and boat questions except to say that I read here on DIS from one poster that the doubles had to be folded or only fit in the handicap car which of course can only be used when unoccupied by a handicap person needing it.
Are u planning on having the double and renting for the 6 yr old? If not, u might consider what we did very sucessfully on our trip last month. We have a 4.5 yr old, a 22 mnth old, and I am 7 mnth pg. So only 2 kids, but this could work for u if you have the 4.5 & 6 yr old alternate riding. We bought 2 single Mac Triumphs. Loved them!
I think your toddler would be comfortable, they are soooo easy to maneuver, the rain cover is excellent and kept them very dry during our 1 day of rain, it has a sunshade and reclines, plus has a storage basket. THey are lightweight, easy to fold quickly, and have a carrying strap. We bought stroller connectors in case I was left alone with the 2 strollers, but ended up not using them bc they are so easy to maneuver, and the older child could always push one a short distance to a bench or table if needed. It was so much easier to have singles in the crowds and buses. And on the resort, we only used the one single for the youngest.