The mast post should take the load directly to the keel or some sort of support underneath sole. If the mast is indeed directly on top of the post and the post is supported underneath the sole, there should be no sag. Assume the plywood bulkhead is attached to the post so it can't bow under load. That leaves the mast support settling. Have you/can you see under the sole to where the post is resting. My w32 had atwartship pieces fiberglassed into the hull that supported the sole and the mast step. If the 28 has the same arrangement, these supports could rot
and/or sag allowing the post to compress the supports and deck to sag.
You are really going to have to attach the corbels solidly to the mast support post for them to have any strength to take the load on the deck. A piece of 3/4 inch plywood on each side of the support post thoroughly screwed into it and a filler in between lag bolted or through bolted to the post would probably be necessary. You asking these Corbels to take the downward force of the mast which is considerable.
There is some reason that the cabin top is deforming that shouldn't be. I'd find out what is causing it before I'd mess with Corbels which probably aren't necessary. The deck core sandwich should be thicker in the area of the mast step. That should prevent the deck from deforming around the post and pushing up into the deck. Check that first. If the deck is uniformly sagging, then the post and its support has to be at fault.
Measure that the mast is indeed on top of the post. Westsail installed the atwartship bulkheads that were supposed to be in line with the mast about 8" farther forward on my boat. Didn't discover this until the interior
was fully finished, chainplates were in place for the as designed mast location, and the step was in place. It seemed to work out okay but always made me very nervous. If we'd kept the boat and taken it on another cruise
, would have installed a SS post directly under the mast and through to the keel.
The workers at Westsail weren't shipwrights and had little understanding of the physics/reality of a boat going to sea. They did what they thought they were told to do. Most did it well but sometimes the QA didn't catch the errors.