Your post made me realize they have OUAT scheduled 45 minutes after park closing. That is very rare/unusual. Really can’t believe those August days weren’t moved to at least 10pm (and part of me wonders if all of those times stick). But anyways...
HEA is an 18 minute show, so let’s round up to 20 just for simplicity. At the Terrace, once the show is over things are generally going to be packing up/wrapping up as the party is effectively over at that point. YMMV on if they let you or not, but I suspect sticking it out from 9:20 to 9:45pm may be stretching it a bit.
The After Party dessert area is in the upper portion of the Tomorrowland Terrace. So, you’d watch HEA, head to the dessert party area 9:20’ish where you can chill/partake in the desserts. You can then head out to watch OUAT at 9:45 if so inclined. Some may want more dessert party time, for some 20’ish mins is enough.
Does that all make sense / address your questions?