"I still want to know why the hell Desann says 'The death of Katarn's woman blah blah blah' to Tavion when it's just the two of them on the shuttle! "
Because it was the death (i.e. the illusion of her death) that motivated him to reveal the valley. It's the same kind of star wars trickery we saw from the old Obi wan (" vader was a former pupil of mine that killed your father, luke" not really a lie, just a twist on the facts, since the birth of vader was the death of anikan).
Btw- who thinks Tavion will be the big baddie in Jedi 3? I personally found her fight to be the hardest in the game, she was DEADLY in close quarted combat and much more stylish than desann (who had a killer grip, but that was about it).