What was the point of the View Master toy and slide projector?

Yes, a link to their childhood. In the first swimming scene at the pool, the swim mask reminds Martín of the viewmaster (you see him put it up to his face happily reminiscing) and, specifically, the hawaii slide and pineapples. In the scene where Eugenio rejects Martín, Martín is again wearing the swim mask -- a callback to the earlier scene. After Martín is gone, Eugenio consciously understands the import of their earlier childhood connection and, in his writerly mind, probably sees their love as inevitable, destined. The view master, hawaii (a sanctuary, a magical place, a shared memory) and the pineapples are all symbols of this. The shot of them laying the field is a direct allusion to the pineapples. (I think the projector was just a way for the director to show the audience the slide as well as Eugenio's face as he realizes what he may have lost.)

This film rewards multiple viewings.