I think it would be much easier (and better looking!) to have the negative cutouts actually part of the lowpoly! Good luck, this piece could look very cool!
The biggest problem here is you've got holes and gaps in your mesh. The renderer doesn't know what to do with this sort of data, and its going to raycast onto something random. In your "This is how it should be" example, you have those pink normals which are just facing in some random direction, both methods you show there…