To which Nate French has answered :
She initiates as a triggered effect, not in play. Thus, she cannot be canceled by Bloodriders. (1)
Her effect does initiate before she is blanked by Fortified Position, and the then effect will continue to resolve. (2)
When she chooses targets for her "then" effect, Meera is completely in play, and this aspect of her effect resolves as an ability. Viper is immune to her "then" effect. (3)
Death by Payne responds to the entire triggering of Meera's effect, not only the "then" aspect of it. Since the effect was not triggered as a character ability, the card Death by Payne cannot be played in response to her. (4)
The "then" aspect of Meera's effect was still triggered (it is part of a bold Any Phase trigger) so Joffrey is immune. (5)
Just for information, it has been ruled in another post that City of Shadows and Hidden Chambers do not apply to Meera's effect, but only when she is brought out of the shadows through the normal process.
