thats why i suspect a broken crank since that nut/bolt under the recoiler is what holds the flywheel on the top end of the crankshaft and should turn in unison with the blade
what i am talking about is, with the recoil off the engine, you should be able to turn the engine from the top by using the flywheel. thats what im talking about when i say the top and the bottom should turn the same amount of effort since they are one solid shaft sticking out of both ends of the engine. if the blades spin easy then so should the flywheel but if you have some sort of built in clutch or brake on top of the flywheel, that sounds like where you could have an issue, but the blade should also be stopped by the same flywheel clutch and/or brake
bottom line is, remove the clutch brake thing on flywheel and then see if its free, that will tell you what to do next