I can only answer part of your question. We just came back from a split stay, but we did NOT do the dining plan for the second part of the stay, only the first 3 nights, so I am not sure if you will have to do two separate ressies with separate park passess to get the dining for the entire time. Seems to me if you book everything together you should be able to get the dining and park passes all together in one purchase, which would definately cut down on the ticket prices.
Anyway, when you switch resorts, you will get a new room key. You have to take your new room key to the conciere desk to have your park passes transferred over (and dining too if you are able to do it all together, I suppose), so that you only have one card to carry around.
I hope that made sense. Good luck.