I am okay with Velanna coming back, even though she was one of my least favorite characters. She was MIA, but not dead if you left her at Vigil's Keep. So I'd be open to the idea.
I really don't like the fact that Leliana is retconned into DA:I.
Er, she wasn't though, unless you are referring to possibly "killing her." She has been the Divine's spy and bodyguard for years since the end of Origins. Plus, she was also helping Cassandra as a Seeker of Truth to look for the Warden as well as Hawke. Even in Origins, when you recruit her, she wears the Seeker's Amulet, which should be a big clue. The Chantry member she saved at the end of Leliana's Song was Revered Mother Dorthea, now known more famously as Divine Justina V, said to be one of the greatest players of The Game.
If you were one of the players that "killed" her, it was most likely in Andraste's Temple at Haven. Which is full of lyrium, holy spirits and sacred ash wraiths. If it was shown that she was decapitated, that was an in-game finishing move and not based on the story. If you "killed" her, yes you saw her HP drop to zero and watched her fall. Who's to say that the Warden only thought Leliana was dead?