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.
Source: TPCi Rules Team (2017-08-31)

Relevant cards

Note: All applicable cards may not be listed below.
SM3_139
SM3_158
SM3_88
SM3_88a
SMA_SV70
SMA_SV70