First, the hero would need the gem. Second he'd have to know the special word to summon it. Third, the phylactery would have to be unattended. Since Redcloak always kept it on him, the spell would fail. All the hero would get is the knowledge that Redcloak has the phylactery and the location of Redcloak.

As for the anti-scry bit. I'd say that is a good idea. O-Chul only knew about it because Soon Kim identified it during the battle. The OOTS had no clue about where the phylactery would even be until O-Chul told them. If they tried to scry out the phylactery, their attempts would fail. They'd need some epic magic to try and learn anything. Possibly a trip to the oracle.