@pjmac, If I understand correctly, so 1 central leader with smaller leaders (suckers?) coming up from on either side, from below the soil line. If it’s below the soil line and the grafted Castagnat leader has roots coming off it, I would be very tempted to cut it off the main tree and plant it on its own. I think you would change it hormonally enough to possibly start growing again and direct all energy towards its now one and only leader topped with the Castagnat.
Also kind of depends on how attached you are to this I-258. You could also stump the central leader. It might serve as enough of a rejuvenation prune to again change hormones and promote growth. That leaves the 2 grafted side leaders to soak up all the resources.