Gary,
I had some sensitivity in my chest area the first few months after my CABGX5 seven plus years ago. This included a periodic slightly pulled left pectoral muscle type feeling, which I attributed to scar tissue formation where the surgeon had harvested an artery for use as a graft. It also included occasional sensitivity around the scar. All of this disappeared over the course of months.
One thing that never disappeared was a heightened sensitivity at my solar plexus, including a visible bump that didn't exist presurgery. My doctor could not explain it and I speculated that it might be related to the wiring of the sternum. I never pursued the issue as it never bothers me unless I directly press on my solar plexus. Moreover, it doesn't bother me when I lift weights (and I lift heavy although, at 75, what is heavy for me now I would have considered baby weights twenty and even seven years ago). It also hasn't interfered when I do calisthenic exercises (pushup variations, dips, pullups). I suppose I wouldn't be able to wrestle anymore, but I gave that up at 37, so no big deal.
You mentioned seat belts. What I did and am still doing after seven years is to place the shoulder harness portion of the seat belt under my arm so that it doesn't press against my sternum. I have no idea how or to what extent this affects seat belt efficiency, but I personally don't care. You may feel otherwise and, if so, you can research it.
Anyway, as a non-doctor fellow CABG survivor, I believe that what you are experiencing is a dynamic process of healing and or surgical related changes that is a subset of the overall process of change, including aging, that our bodies undergo. I find it useful to think of my body as a fluid, changing process rather than as a static thing but, again, that's just my personal metaphysics and you may disagree.
I wish you the best in your healing process.
Ira