The rules specifically state that if a cost is prevented or otherwise unable to be paid, the action has no effect. I can't make that any clearer. The rulebook then gives one such example with Grishnakh. That doesn't mean that ALL such preventions have to be done in the same manner as Grishnakh; he's just one example.
Elessar's Edict falls under that same heading. The cost of
SFTF is placing a companion in the dead pile;
Elessar's Edict discards the Wraith instead. Therefore,
SFTF can't pull the 3
cards because
SFTF's cost wasn't paid.
That rule is already in the system. For example, if
Morgul Destroyer is played and FP chooses to wound the Ring-bearer, but then uses the Ring's text to take a burden instead of a wound, the Destroyer still adds the threats.
When Scouring is used, it prevents the Shadow action that is discarding that
possession. In order to attempt to discard that card again, the Shadow player would need to play a second Marauder.