OK - I can comment on this since I've actually been at 2 WPT tapings here at Commerce.
First - they tape the crowd reaction shots BEFORE they start playing to get some stock footage. They do this incase someone mouths off or does something obscene when they need a "real" crowd moment.
Second - Yes, Sexton and Van Patten are commentating live right there as the play is going. To my knowledge, they are not told the hole cards. I know they do not have any monitors at their desks, but they are plugged into the sound system. You can hear them, which again makes me think they are just commentating as they go, and then add in more commentary in post production in regards to the hole cards. That way they can get some of the excitement of the "live commentary" and then blend in the post-production commentary knowing the cards.