It’s intended to be a surprise to the Resistance, certainly. But remember, the briefing scene comes directly after Hux’s psychotic rant against the Republic and the senate in particular. So in juxtaposing two conflicting pieces of information, the audience will hopefully understand that the Republic is always the true target, with the Resistance deliberately misinformed so as to lull the Republic into a false sense of security.

As for the problem with blowing up the Starkiller, I assumed that it would have multiple uses after absorbing the star, it is logical to destroy the planet. Also, nobody has called off the Resistance attack, and why would they? Even if the planet had no further shots to fire, there is still a massive amount of FO equipment there.