Sounds like it'll be a good trip. I'll be interested to know what the temperatures wind up being and how the PR does. If you're really diligent, you can use a windshield on a PR. The windshield will block the wind so the PR will be more effective but also trap some heat that will warm the canister. You just have to keep checking the canister with your bare hand. If the canister starts feeling hot, you'd better back it off quick.
As for putting the canister in water, warm water will work better, but any liquid water will be better than nothing. You could start with some of the water in your Nalgene in a pan with the canister and some in the pot on the stove. Heat what's on the pot. Pour the cool water in the pan back into the Nalgene and then pour the warm water in the pot into the pan. That way you have something to start with. As I think about it, if the water in your Nalgene has been in your bag with you all night, it might be warm enough that you don't even need to heat part of it.
Anyway, experiment with it. As long as it's not super cold, you should be able to get by with a PR, especially if you've got another person there with a second stove. You could always get them to heat you up a pan of water, and then your group has two stoves available which could be handy depending on the size of the group.
Good luck and have fun,