K.M.
Okay .... first - you and your husband need to talk. You have to work together and be a family. Threats never are a good thing.
As for the travel - I would do the first trip alone and have the nanny stay with the baby. Do you have enough stored milk so that you can freeze it while in NY and just bring it home? The second trip try bringing the nanny and baby. My only reason is so that you can see how the first one goes as far as time. When my girlfriends travel - they work from 7 or 8 am until after dinner time because they are required to be a work dinners. I would hate for you to pay all the additional money and have you be so exhausted that you struggle to function. Once the first trip is done, then you have an idea of how the time will pass.
Good luck - you will be great and so won't your baby - we tend to miss them more than they miss us!