Jon and Meera being twins isn't unreasonable, but would have a weird impact on Rhaegar's prophecy thing (he was looking for 3 kids, not 4).
Meera being adopted into the house when Jon didn't isn't unreasonable, the Reeds are known as a bit of a weird house, and the current lord's wife doesn't even have a house name so may be even be lowborn? Reeds are weird.
Blackfish not being there, could be a lot of explanations. He could have just wanted to leave expecting Edmure to make an ass of himself. Then again, a unmarried 60 year old, who fought with his brother for decades over a reasonable marriage arrangement with no children, yeah, him being gay would make sense. Which I never thought about. That is kind of awesome, I never thought about that before. Yay Blackfish!