We know that all grace following original sin was given through the merits of Christ the Redeemer. What about prior to original sin? Was the grace in which Adam and Eve were created given based on the merits of Christ the Redeemer? (If so, I'm not sure how you can get around the Scotist doctrine of the Primacy of Christ...)

A Thomist friend of mine asked me today and I didn't know, but I said I'd try to find out. I looked in the Summa where St. Thomas talks about man in the first part, and where St. Thomas talks about Grace in the second part, and where St. Thomas talks about the effects of the Passion, but I couldn't find anything ...

The Thomist version is no, Christ did not merit original justice for Adam and Eve. The Scotists say he did

what is the reasoning behind the Thomist position?

Prior to the fall, Adam and Eve were totally sinless and did not require being saved. It was only when they went against the will of God, and committed the Original Sin, that they, and all of their descendants, required the salvation of Christ.

This is very straightforward, and not at all complicated.

I'm afraid it's not that simple. Adam and Eve did not need to be saved, but to attain to the supernatural destiny of man, which is unity with God, they still needed the gift of sanctifying grace. The OP is asking whether that grace was given in view of the foreseen merits of Christ, or for some other reason.

Right. Traditionaly we distinguish betwee gratia Christi and gratia Creatoris. The grace merit by Christ, and the grace given by the creator prior to original sin. For Scotists even this gratia creatoris is a gratia Christi, Christ merited those original graces, as well as the graces given to the angels.

But in the Thomist view, as the Incarnation was predestined based on the need to redeem man, we distinguish the grace of Christ from that of the creator. The latter being simply the free gift of God to His creation. Though unworthy, the unwothiness was merely negative. We didn't earn it or merit it. But after the fall the unworthiness was positive, we not only did nothing to deserve it, we did things to make us not worthy.