If you talk to Simon, he says he never even knew Tara's full name (that's just tragic). What if we have to Dive to the bottom of Tanzan Lake and pull her ID, so that he has something of hers, and knows her full name, then helps us infiltrate like you said. We should also be allowed to retrieve her Pokemon and give them to him, so that they aren't trapped there forever but at the same time they're with someone they know and trust.
Plus, a PULSE Darkrai is most likely the cause *shudder* but how would the player get close enough to battle it without falling asleep? Continually throw Rage Powder in their face every time they felt sleepy?