Grant was someone she would never, ever be tempted to involve in any alien stuff. That was his attraction. (Unfortunately, his digging would twice involve him in alien stuff, the second time to his death.) Even so, it doesn't make a whole lot of sense. I don't think having Isabel suddenly attracted to older men has any reason for it other than some producer or writer's perverted fantasies. And it's noteworthy that Alex and Isabel's relationship was so well-established by the end of season 1 that when they decided to go for the angst in season 2, they had to have them break-up offscreen beween seasons. They couldn't think of a way for it to happen!
There are any number of ways they could have kept Alex, or brought him back (enough so that credibility was stretched further and further every episode it didn't happen).
Personally, I prefer relationships that are in defiance of their pre-planned "destinies."
As far as leaders... they never accepted a leadership role or obligation. It isn't a betrayal to not go to Antar, because they never made a commitment. An argument might be made that Max made such a commitment when he began exercising his royal authority, but he didn't know what he was getting into. The obstacles are overwhelming, and as Kathy says they are teenagers.
