Wow! that's an awesome archive / resource!
Especially as the vessel you are looking at is not perfect (LOL!), as someone new to all things boaty this should be invaluable to you, it is always more useful to be able to talk with folk with the same vessel (their are always "issues" (quirks?!) which are specific to each vessel), and preferably someone who has already done the same jobs. (BTW not trying to get rid of yer!)
As I said in one of my previous posts, I am not hands on familiar with drop keel boats, but anyway.......
It may well be that the mechanism is stuck, rather than (or as well as?) the keel - so maybe worth trying to disconnect this? Drop keels got in there somehow and if their is no access from the top - the bottom is where it went in
, so once disconnected from the mechanism it should be removable the same way. I would do lots of research
and close inspection
of what you have so you clearly understand what you are trying to do (and have in fact released all the fixings??!!).
With a bit of luck the keel might be stuck with "only" a combination of some rust and a lot of marine growth. and maybe a few pebbles thrown in, if she was kept on a drying mooring
Maybe try with a hosepipe or a pressure washer, maybe also using detergent? and then a long flat stick / metal rod to see if you can dislodge / wash the rust / cr#p out plus a bit of "gentle persuassion" and then the application of "brute force", if needed. I would be surprised if it would not come out (eventually) and you needed to cut into the centre board case or it had done any serious damage.....but of course no guarantees........
The problem as you already know is their is only so much you can try (if anything) before the boat is yours.......I don't know if their is any easy answer to that - maybe be prepared to risk your $250 by telling the owner that you will buy if you can free the keel beforehand, unless you break anything and then you buy it! (in any case I would be surprised if you could not E-bay the entire vessel or some of the parts
for at the very least $100.........I would risk it even if the owner does not let you have a go at the keel beforehand, but of course it is not my money
Whatever you do, when you find yourself peering up the centre board case with a torch and then poking things with a stick, make sure that their is something substantial apart from you to catch the keel when Gravity finally takes over.
Once you have got it out, can you fix it? I would be surprised if it was that far gone you could not - a Keel is deliberately a big lump, should be plenty left over once cleaned up. But don't quote me on that!