1. Does OL decide the number of gems to assign to guard benefit before or after the hero role for attack power?
Heroes' book (revised), page 11: "When a hero is attacked, after the attack power is determined, the hero may defend against the attack." The same applies to the OL.
2. Crossbowmen - have guard icon attribute instead of armor icon attribute. Does OL receive one die for guard for free or does OL need to allocate gem/s to Guard benefit and only then he will receive the guard die in addition to the dice from the guard benefit.
ALL of the add-on units misprint the armor icon as the guard icon. This is a misprint.
3. Just wondering if Hadrathus can commit suicide by using lightning storm on his area.
Yes - the damage does not spare any units, and ranged combat can target one's own area (the rules say so by saying that a unit always has line-of-sight to its own area).