Who says whoever it is dies in 521? Who says the person dies in in the Underworld? And in the grave yard photos for the funeral, you clearly see Hook but you think it could be him. So, your logical doesn’t really follow.
Not to mention, we’ve been hearing that some scenes from 523 were already filmed ahead of time to schedule around Ginny’s Zootopia press schedule. Another thing that points at this is that Amy Manson specifically said that she was returning for the finale, and she filmed as Merida last week. So yeah, the funeral scene filmed last week very well could have been for 523, not 521.
Amy (Merida) didn’t say she will only appear in the final episode, she just said she will appear. So clearly she may appear more than just one.
And besides, just because we see Hook’s actor in this funeral, doesn’t mean his character is alive. I’m not jumping to any conclusions but one possible explanation for that is that he is a spirit appearing from his tomb and saying goodbye to all (Just like the spirit of Fergus did for Merida). And Have you forgotten about that photo of his headstone in Storybrooke? How do you explain that?
Just because you here about scenes were filmed for 523, doesn’t mean it is true. Often times people are misled for what they heard. And usually it ends up wrong. Besides after the Only You title came, they have been filming the new york scenes and forest scenes after that. And Funeral scenes from any episode usually start at the beginning. Some of the previous episodes had alot of filming and photos, and those filming could be for ep 21. We’ll just have to wait and find out when it airs. You remember that the title for ep 21 is Last Rites? It certainly relates to a funeral or someone dying. Which might explain the funeral.