Use year determines the month of each year you get your points and your banking dates. It has nothing to do with when you can go (anytime) or when you can call to make a ressie (11 months in advance at home resort, 7 at others). There is no particular most popular use year. Because of banking dates, those who go the same time every year often prefer a use year that begins no more than 6 months before the month you usually go so they can bank all their points in case they have to cancel.
If you buy from Disney you will pay dues on a monthly basis beginning from the date of purchase but with first paynment due shortly after closing; thus, yes, it is prorated. If you buy resale, any dues still owed for the rest of the year must be paid at and as part of the closing.
The Disney Collection is any Disney hotel other than a
DVC resort avaialble by use of your points. The points needed are high; for example a regular room at GF can cost more points than a 1BR at a DVC resort.