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
![Gondor [Gondor]](https://lotrtcgdb.com/forums/Smileys/classic/gondor.png)
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
![Shire [Shire]](https://lotrtcgdb.com/forums/Smileys/classic/shire.png)
possession. In order to attempt to discard that card again, the Shadow player would need to play a second Marauder.