The MK dessert parties are normally bookable at 180 days, although sometimes it jumps off this pattern. They just recently stopped becoming bookable 2/1/20+, the first time it has been off the 180 day pattern in a long time.
I'm not sure about PP photographers in the Terrace, I haven't been to that version of the party in awhile.
OUAT is visible from the Terrace, but it's just an OK view - not great. Depending on where you are in the Terrace, trees may block some lower projections. It's passable though, in my opinion - in the same way the Terrace is passable for HEA viewing. You may miss some of the extreme lower projections, but it it's still a good show.