I'm no mechanic, but have you cleaned out the carb to make sure there isn't any gooey gunk left in there that can stop the flow of gas? To help prevent this, I add stabilizer to the last can of gas I buy at the end of the season. My mowers always start up in the spring. Beats cleaning out the carbs. Good Luck, Vickie

It sounds like the device that holds your clutch IN, and acts like a parking brake, is activating every time you push the clutch in. You should be able to see this if you follow the linkage. Have someone push the pedal in and see what's happening. It sounds like you have a safety switch problem. If you can make your starter work with your battery, the problem is probably a safety switch or maybe a bad ignition switch or wire that fell off at the switch. Check your fuse too. Be sure the unit is in neutral, the clutch fully in, the PTO turned off. Try moving the shifter, or pedal, a little bit off neutral with the key turned to start. If you have a neutral safety switch, you may need to go slightly towards forward and reverse to make contact.

It sounds like your starter isn't engaging the flywheel. Do you see the top of the motor turning?

Pull the valve cover and check the valve clearances. The engine not wanting to crank over may be due to valves needing adjustment to restore function of compression release. Don't worry about the flywheel key shearing, that is not something likely to occur on a mower that has a belt driven deck, The concern over shearing a flywheel key by hitting something and stopping the blades is only true in push mowers where the blade mounts directly to the crankshaft. Belt driven blades can't stop the crankshaft quick enough to shear a flywheel key. The puzzling part of the description is how the exhaust seems to have gotten louder just prior to backfiring. Start with the valves.