My leaders told our cast this during track talk a few weeks ago, so yes, this was before offsite can prebook. However, it was when MyMagic+ was opened to all of our Passholders. If the number changed, my cast hasn't been informed.
Once the FastPass+ entitlements are able to be reserved, they will remain available until they are booked. There are no adjustments at the 30 day, 45 day or 60 day periods. I'm sure some Passholders may have been having difficulty making a FP+ for Anna and Elsa when their window just opened. The 45 day window is not currently attached to any types of tickets, but it can be enabled at any time. When and if, nobody knows. This is Disney. However, it is built into MyMagic+.
The numbers may seem high, but they are needed. In theory, this allows guests to make changes throughout the day by visiting a kiosk or using their mobile app. This also allows most onsite guests to be guaranteed three experiences throughout each day. Our day guests and those who don't know they are going to Walt Disney World until they are in the car also will be able to select some FastPasses at the kiosks. Not to mention, our Cast Members are not able to prebook at this time.