But if you are purchasing from Disney, can you pick your UY or is it the month you sign your contract?
it has nothing to do with when you sign your contract. it just depends on which use year they are currently selling.
if they have your preferred UY available, you can request it and they will sell it to you. otherwise, you could choose to waitlist to get a certain UY.
(and unless you are certain that you will be traveling in the same time of year for the next 35-50 years...UY is really not that big of a deal...)
also, if you start with one contract UY and then buy another contract with a different Uy can they be combined?
if you already have a ("master") contract, DVC will only sell you add-ons with the same UY. (unless you buy another (160 or more pts) master contract.)
points from 2 contracts with different UY months cannot be combined at all without using up the once-per-year transfer for both contracts. then they can be combined at 11 months if the same home resort, or at 7 months if they are for different home resorts.
without using the transfer, you would have to book individual nights separately and link the reservations.