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 are bad.
To get this result with a simple lowpoly, you want a fully capped mesh, no holes etc. The renderer can't read your mind to fill in the gaps. You need to think about this in terms of what the baker is seeing, then why you're having issues makes perfect sense.
So, we start with a capped mesh:
Assign mat colors to the "holes"
Model simple lowpoly
Bake normals+rgb, and make sure the holes are completely black in both diffuse and spec:
So, not really a baking problem, aside from having unrealistic expectations of how something will bake, but more of a modeling problem. Its the content the baker is looking at, not *how* the baker is looking at it.
With a relatively small part of your model, plus that fact that it would be unlikely to be seen up close(or at all) in FPV view, using this sort of "fake hole" workflow is the simplest solution. If its for a portfolio, modeling a more interesting lowpoly might be a good idea, but you still need to cap those holes.