We had a rough experience in front of the castle.
We watched HEA during a trip to Disney in August. I think we were there the night of the 21st. We walked over from Tomorrowland to find a place to view, and had a really difficult time. We didn't get into a spot early- we had been on a ride just before, so had very few options for viewing. Basically, poor planning on our part. But, cast members were trying to clear the streets and kept telling us, "you can't be here", but it was literally so crowded we couldn't get anywhere- even away from the viewing area! We should have done an about face and walked back into Tomorrowland, but we were initially trying to go away from that area. In hindsight, it would have been a way to escape the thick crowds, but we would have sacrificed the show. By the time we walked a little further into the crowd, we were pretty much trapped (aside from heading to park exits), so tried to make the best of it. We did end up on the corner end of the block where the ice cream parlor is (forget the name). A nice couple from Manhattan let us squeeze in with them. We were able to see OK from here. It was very stressful, though. If they didn't let us squeeze in, we would have been pushed to the exit by the cast members.
Once the show was over, we wanted to walk deeper back into the park since we were not planning to leave until the last kiss goodnight, but there are so many cast members ushering the crowds towards the park exits, it was almost impossible. At one point, a cast member was literally blocking our path from returning into the park and said to me, "you can't come this way- need to keep moving in that direction" (pointing towards exit direction). I pointed out that the park was not closed and we were not leaving, so did not want to follow the huge crowds down there. I understand why he wanted us to keep moving- the crowds were so insane. But we were also not being pushed out early. We drove 20+ hours to do this vacation- no way were we leaving early! I was seeing that Kiss Goodnight! Somehow we managed to fight through the crowd across Main Street with our strollers (disaster) and get back to the path over by Crystal Palace. Once we reached that area, it was crickets. We were totally fine and we were able to head back into the park via Adventureland. In case you are wondering why we didn't just stay put in our spot until the crowd thinned- we couldn't. CM's were clearing the area and we had no choice but move. Just not in the direction they wanted us to move.
Moral of the story- have a plan and find a spot early! If you are planning to leave the park right after the show then it's probably fine to watch from in front of the castle. If you want to stay later, find a viewing area where you can make a reasonable escape back into the park (and when you find that spot please share it with me!). Another thing I thought after was that we could have just ducked into a store on Main Street until the major crowds thinned, but that would have eaten up a lot of time. Not sure I will watch HEA again, but if I do I think I will try the Adventureland side of the park for viewing.
Good luck!