I didn't quite get what was broken, but it sounds like you might have hollowed out the counter piston too much?
If it is aluminium it can be made solid or close to it, the extra weight would not be noticeable.

25% ether is a little low, you need closer to 35%. 25% with all castor oil should be fine and the rest kero, preferably there should be some ignition improver too. It is the ignition improver and high ether contents that keeps the compression level low enough to avoid damages to the engine. The compression ratio only needs to be slightly higher compared to glow fuel.

I don't have any such large engines myself, so I cannot comment on the prop size, other than that it is good to aim for max revs of around 10krpm. A higher load is not a problem but a smaller prop will increase the load and wear on the engine.