I've actually been to WDW with kids a lot. I have three, and when the the youngers were too young to go, then one of us missed on the ride while the other rode with the one who was old/big enough. I didn't know about Rider Swap, so i didn't use it. And it's not a matter of bringing extra kids.
But let's be clear about what's being discussed. The steps that I see the OP is talking about are as follows: 1) Go through the line with the 1 adult having a FP for a T1 ride. Getting to the RS area and saying that the 20-month-old can't ride, so can we get a RS pass? Riding, then getting the RS pass so the other adult who did not have a FP for the ride rides without one. 2) Repeat the procedure on the other T1 ride they are interested in, so everyone has access to FP rides for both T1 attractions. I'm guessing that the daughters either split up or have one FP booked for each ride, otherwise I don't see how the plan works, but that's not the point. What it looks like to me is that the OP gets 10 rides out of four FPs, and their party gets access to Fps for 2 T1 attractions instead of one
All I'm saying is that this service wasn't designed to let people get around certain rules. FP rules are distributed -- everyone gets a certain number, and everyone gets a certain number of T1 rides. The way the OP is discussing it, each member of her party gets 2 T1 rides and an extra FP. Not a big deal in the scheme of things, but you'd have to see the inherent unfairness in that, don't you? I know WDW with kids is a pain and frought with inconvenience, but so is everything there.