This, I can mostly agree with. The only thing is that, as you said, it is extremely unlikely, not necessarily impossible. Considering how such things always turn out in role-playing game storylines, the near-impossible always managing to manifest in some way, it almost makes it certain, albeit in some ironic manner.
On the whole though, I actually support your theory in that the pendant just came into possession of the Guardian ancestors, rather than somehow Schala being Marle's actual ancestor. It's a far more feasible, not to mention more easily explained theory than Schala and Marle being related.
