there are a few potential "outs"...
without having to go down the "impossible girl" route of survival...
We know that the doctor knew about the message long before the medieval tourney. He's manoeuvred the ladies and the tardis to get picked up by snakeboy at the time of his choosing.
"did you suspect it was a trap?"
"I still do..."
Not sure who is being trapped by who at this point though.
A previous posted noted that both ladies were wearing handy time machines, begging the question - why were they effectively jumping up and down shouting "shoot me?"
From a plot point of view, I'm expecting more from the invisible planet thing. That's such an awesome concept that it demands more use as a plot device, and not just a way of dropping the characters onto a planet they supposedly weren't expecting. If this one wasn't going to be used later, moff would have just had the daleks teleport them there directly, saving that further star wars reference to hyperspace.