You talk about having an adult stoker ride the bike on occasion. and leaving the kiddy cranks in place.

Here is how I set up a similar system to swap my bike between commuter mode (2 kids one with kiddy cranks, one on trailer-bike), and race mode (adult male stoker).

I have my kiddy crank chain set up on the outside of my regular sync chain. This means that undoing the clamp for the cranks lets them and the chain come off in one piece. My kiddy seatpost has the trailer-bike attached to it, so I simply remove that entirely and replace it with his seatpost (marked with tape to his height) I also have a second pilot seatpost with drop bars/short stem on it, rather than the rise bars/long stem on my commuting seatpost. This way it only took a few minutes to change the bike over and there was no need to faff about with pedals, since they stayed attached to the cranks.

Cheers,

Cameron