I did my apprenticeship at removing broken Gibb Head keys from the McDonald Super Imperial Crude Oil engine I did up not too long ago. Both sides had broken heads and were also broken off flush to the flywheel. So spent the next 2 days drilling out the metal using 1/8" diameter drill bits. Had a stuff up when one bit bottomed out on a taper on the crankshaft I was unaware of so that snapped off inside too. It was a true challenge to get these buggas out but I did win eventually. Then spent a bit of time using wet n dry paper polishing the crankshaft so when or iff the flywheel was going to move and come off there was no rust to hold it back. So with a hydraulic puller, a lot of tension and then heat they finally came off. Total time spent removing 2 flywheels was close to 2 weeks, a lot of skin off the knuckles, a hell of a lot of swearing and an ongoing apology to neighbours for my language. So I do feel your pain on this topic.
As for Gibb Head keys, yes there are many different sizes but now not all are made. Engine manufacturers used these keys because they did the job and were cheap to buy in at the time. My McDonald has odd size keys so had to mill them down in height. I am not a machinist but a mate offered me the use of his mill so no time like now to learn. I lost hours of sleep working out how to machine these down and keep the taper. The solution when it finally came to me in the middle of the night was to place both tapers together and this left the flat sides that are now parallel so was an easy milling job. In order to fit Gibb Head keys they nearly always need some filing using bearing blue to ensure the correct fit. Fortunately one needed a little drawing filing and the other was a bit more but not hard to do just messy.
38ac, where your pics of the keys 'naked'?