With nothing to lose...
I'm sticking with a weak battery as part of the problem.
Whatever caused binding in the cylinders (gas, oil, mouse nest) was too much for the weak battery.
As Top-Water has stated above...
Hydrolock occurs when gas leaks into the cylinder.
Some mowers have a valve that keeps the gas from flowing until the key is turned on. This helps when the mower is bouncing along on a trailer. If this valve is shot and the float leaks or is stuck open, you can put enough gas into the cylinder to cause a lot of trouble. The best one could hope for is the engine won't turn. Under the right conditions, (strong battery, strong starter, piston positioned just so), damage can be done by the piston compressing what cannot be compressed.
One way of knowing if gas leaked into the cylinder is to check the quantity and quality of the crankcase oil. If you gained oil, it's likely contaminated with gas.