cccdisney, are you aware of the altnerate viewing area for Nemo? You can see the "movie" of the ride, but it's not in a sub. If you want to try that the next time you are with your 4 year old there, ask the CMs about the viewing area, so you don't *have to* do the special swap (unless you want to).
Most of the lines I can think of, other than Nemo, are very narrow. I don't even know HOW a person can shimmy their way past people. The one time I saw someone leaving the line due to a potty-needing toddler, she was very slim and tall, and was able to climb over the "wall" in the Peter Pan ride. I would not have been able to do that, and those lines are so narrow you would have to be "narrow" and those you are passing by would need to be too.
OP, when DS was 2 we had my brother there to amuse him, WE amused him, and for the times when he couldn't be amused, we held or wore him in the Ergo until it was time to get on the ride. He did use the stroller a bit from ride to ride, and for a nap.
Here he is in the Ergo:
I can't recommend things like that more for 2 year olds, though they do require practice and for everyone to get used to. I wore him from when he arrived, so we're all used to it.