JenLily is correct about how it all played out with the "Chinese killed Tara" story. I will just add that Juice is indeed the one who gave Gemma the information on the Chinese and came up with the story to pin Tara's murder on them. Gemma thanks Juice for the story when she talks to him at Wendy's apartment. She makes the comment that she owes him for giving her "the story."
Gemma must have told Jax the story when she visited him in prison prior to this episode. And yes, when Jax was asking the club if they were all in, he was referring to the fact that he was about to start an open war against the Ling crew. They agreed and the rest of the plot continued. There's a moment after the meeting with Ling and the Mayans when Jax tells Chibs and Bobby that "things were happening more quickly" than he had thought they would. So yes, the club knew what was up because Jax told them during their last "church" meeting near the beginning of the episode. Also during the party when Jax asks the Reapers to pick up the random member of Ling's crew, their president demanded to know why. That's when Gemma tells him and us, the audience the story. But if you look at Jax and the other SAMCRO members none of it is news to them. In fact, they all look sad and haunted that they have to think about Tara's murder at all.
ETA: I also can't stand the thought of Wendy raising those boys. She is not a good mother as she is still very much a junkie who just had a relapse two weeks prior to this episode and didn't complete this last rehab plan. I have a feeling she's going to start using again, once again caving under the pressure of all the secrets. This time she's keeping Juice's secret. Last season it was Tara's but I have a feeling the results will be the same. And we know Nero will try to help because that's what he does; he helps women in need (see Venus, Gemma, Wendy, his crazy sister, and all those prostitutes who he is now pimping because he can at least protect him). He's the new "hero" on the show but he's just as corrupt as the rest of them, which we've known since he admitted to Jax at the end of season 5 that he had the money to buy the farm and retire with his son all along. He just can't leave the game. So he's a fucked as the rest of them.