This is helpful to know. I personally am not too worried about it ... We'll try to arrange seating so we can least see her but I am uncomfortable with dark rides if she's separate from us. But it sounds like those won't be a problem
I may revisit this question as we get closer and start planning our actual days -- we could always split up and do an attraction at two different times, if needed. The 2.5 year old likely won't care which attractions she misses out on.
On 1-4 seater dark rides, everyone gets their own cart/pod. So a person, be it a child or adult, would never be seated next to a stranger in their cart/pod.
Off the top of my head, Astro Orbiter, Buzz, Autopia, Snow White, Pinocchio, Peter Pan, Mr. toad, Dumbo, tea cups, Alice, haunted mansion, Winnie the Pooh, you would never be sitting next to a stranger. You would get your own cart/pod.
For Pirates and Small World 1 to 4 people can sit in each row of the boat. But if for some reason a person is sitting in a row by themselves, they would not put a stranger next to them.
Big Thunder Mtn 1-3 people per row. One person sitting in a row would not ever have another party seated next to them.
Matterhorn is only one person per row, so that’s not an issue. Same with Splash Mtn, Although multiple parties may be put in the same log, so you might have strangers in your log.
Nemo Subs and Jungle Cruise and Storybook are “group” rides so all guests just sit in a line. Technically, whoever is on the end of your group will be sitting next to a stranger, but your entire party will be sitting in a row together.
Space Mtn is rows of two. A party of three will be split into two rows, and that “third person“ will likely be sitting next to another person.
Indy is rows of 4. If there’s a party of three, it’s possible a stranger will be placed in the fourth seat in the row.
Rise is rows of 4. If there’s a party of three, they will likely get the row to themselves since Rise doesn’t do single rider, and they generally won’t split groups in the multiple rows due to the way boarding works.
Smugglers Run is groups of six. Each seat is kind of on its own, so while you will be riding with strangers, no one will really be sitting right next to you.
Star Tours I don’t remember how many people sit in a row, but it’s like a movie theater. Your group will sit in a line, and whoever is on the end will be sitting next to a another party.
And for the railroad, Casey Jr, the monorail, and any of the various main street vehicles, it’s also not going to be an issue.
I won’t go through every ride at DCA, but the rides where you might be seated next to a stranger are Incredicoaster (2 per row), Goofy Sky School (2 per row), Grizzly River Run (8 to a raft, and they fill empty seats with single riders), Webslingers (4 to a cart, and they fill empty seats with single riders).
Also at DCA Soarin’ and Guardians have long rows, so like at a theater, whoever is on the end of your party will be seated next to another group.
Nothing else at DCA should be a concern.
Didn’t mean to type out this crazy list, but I can’t sleep so it just happened! If I left out any attraction, I’m sure someone will be quick to point it out!