Well, it does certainly explain your handling problems! I'm rather surprised by the photos - obviously with the bottom bit having been missing for some time, it may prove difficult to interpret the progression of the failure as the subsequent rusting may have obscured some of it. I had always felt that there was sufficient metal either side of the void, and that the corners were well rounded. I had a quick look at some old photos of my keel before it was recoated and can't see anything suggestive of anything similar.
I don't have any original drawings of the keel, but have some approximate ones based on measurements from my boat, and assuming a NACA 0012 section is fairly close.
You may find that someone like Vyv Cox [https://coxengineering.sharepoint.com/Pages/default.aspx] may be interested in your problem. He might well be able to advise on interpreting the failure surface, as well as on potential sources for a new casting. I think writing the specification for a new keel to make sure that you get adequate quality in the casting might be an area to spend some time on.
Others may have experience of how insurance companies would work in this sort of situation, but it's not anything I've had to find out about so far.
Good luck with sorting it out. If any of my approximate drawings can be helpful in the initial stages, let me know.
Regards
Dick