If you can post a photograph of the damaged rudder, showing the splits, and how the timber was attached to the rudder that would help me a lot in trying to figure out what happened. If the boat now has a tiller, did it ever have a wheel
on it? How important is an offset key way? (Beyond my knowledge level.)
Were the bent bolts bronze, as well as the shaft? The way they were bent might tell you something about how the forces on them were.
Since, presumably you know all this stuff, maybe you could play the game
, "How else than how I've been thinking about this might it have happened?" I personally find that hard to do, hard to let go of my preconceived notion, but, if those cracks are horizontal, maybe they came from forces on the rudder it was not designed to withstand.....but from the forces of the sea, laterally, instead of vertically from the bottom.
Did the bolts go through pintles and bend up towards the head
of the tiller" or are/were they deflected laterally?
I apologize for not being able to help here; i think it is an interesting problem. Good luck with it.