Unironically, you are working towards the WoG angle here. IIRC there is a statement that barring literal divine intervention, Greater Nightmares do not get to have a happily ever after period.
Gaia and (to extent) GEAR are Divine and near-Divine correspondingly, and the one happy ending we have seen was with Maiden of the West and her Nightmare of the West, whose twist was for former to be killed by the latter's childbirth while still technically granting Maiden's wish to be a mother - and it was Shiva/Sil who were the literal divine intervention in that case.
As for the situation with Reinforce, depending on the exact effects of their Unison (which IIRC either makes them immune to negative status conditions or something similar), managing to extend the duration of Unison indefinitely might disable the lifespan decrease for the duration.