Your Minion survives... somehow
As written, Nonlethal does not require damage to apply. Because of this, you gain the benefits of the spell being nonlethal.
Attacks with this weapon are nonlethal, and are used to knock creatures unconscious instead of kill them
The spell cannot kill the target of the spell, nor can its damage. From the section on creatures being Knocked Out and Dying,
When most creatures reach 0 Hit Points, they die and are removed from play unless the attack was nonlethal, in which case they are instead knocked out for a significant amount of time (usually 1 minute or more).
You gain the dying 1 condition. If the effect that knocked you out was a critical success from the attacker or the result of your critical failure, you gain the dying 2 condition instead. If you have the wounded condition, increase your dying value by an amount equal to your wounded value. If the damage was dealt by a nonlethal attack or nonlethal effect, you don’t gain the dying condition; you are instead unconscious with 0 Hit Points.
By making the entire spell a nonlethal attack or effect, it does not kill the target(s).
Is this unbalanced? Not really. Final Sacrifice requires setup (you have a Minion to knock out), takes an ally (albeit a Minion) out of the fight, and requires you to spend all 3 Actions on the metamagic and spellcast. You could gain the extra 2d6 damage that it does compared to the 3rd level spell Fireball from a weapon attack (or some other Metamagic to penetrate Resistance or add Weakness) or prevent damage to yourself by casting Shield.
As I mentioned in a comment, it was probably oversight that Final Sacrifice was not given the Death trait nor was it indicated that he Death Trait applies to the Minion but not the fire/cold damage. I would absolutely pass this by a GM before assuming you have more spell slots for not-Fireball in-game.