Derren often updates his blog through his iphone, so its entirely possible he could have done it soon after leaving the casino.
As for the stunt, you need to remember that earlier in the episode he told us the casinos have a way of detecting electronic equipment being brought into the casino. How then, did he manage to sneak a camera in up his sleeve?
Also, if he had hit the right number im sure word would have got back to the casino and they could have had him arrested for scamming, or at least forced him to pay the money back.
Being one away is like saying ' I could have done it, but for many reasons i didnt do it '.
Also, think of what would have happened if he hit? How impressed would you have been? Im sure then a load of wild theories like controlling the ball or the croupier being in on it would have come out.
Mentalists often miss on purpose, i think this is an example of that.