*blinks* oooh dear I just rediscovered this posting and will now add my two cents.
I think this pairing it totally possible!! Elsa and Jack would totally make a wonderful couple! And Jack would even have been a spirit for several decades when Elsa was born so he'd probably have figured a few things out. Especially that someone other then a spirit who can make snow happen out of season is very, very rare. And someone to investigate.
The 1840's date comes from the official "The Art of Disney Frozen".