Do we know that Toby was talking to a woman? Wasn't it just his crossfit group chat? He could have been talking to anyone. I don't think there's going to be an affair, I think they're just going to drift apart. He's changed a lot in a short amount of time, and she has closed in on herself. They're coping with the baby's problems in drastically different ways, and independently of each other. Not a good recipe.
Maybe Kate and Kevin are in denial about Rebecca? Maybe Randall is the only one taking it seriously in the beginning and it causes a rift? Could explain why they let her leave on her own in the future scene. Could even explain why Miguel isn't in the further future scene, could be that he was in denial and couldn't come to terms or handle it and left? Of course he may just be in another room or out getting something for that one too. Or it could all be misdirection and the split from Randall will be over something else...maybe Tess comes to Kevin instead of Randall about something?
It was a great episode. I hate that we have to wait for the next one. But i'm also glad, I think the rest of this season is going to be unbearably hard to watch, as it will presumably cover the 9 months in between now and the flash forward.