If I use Darkrai GX's "Restoration" Ability and then somehow it gets back into the Discard Pile (e.g. Parallel City, followed by Field Blower), could I use "Restoration" again to retrieve it from the Discard Pile?

Yes, whenever Darkrai GX is in the Discard Pile, you may use Restoration during your turn as long as its Ability has not been disabled.

Relevant cards

SMA_SV70