First and most important, you should certainly not run the engine attached to the frame until the frame is in good condition. You may do additional damage otherwise.
Kickback is not caused by pre-ignition: pre-ignition only happens when some deposit or surface in the combustion chamber is quite abnormally hot, which won't happen when the engine is not running. You may have been thinking of detonation, which can occur at more normal temperatures, but at least for a petrol engine, it is quite unlikely when performing a cold start provided the fuel being used is petrol. If you substitute kerosene or paint thinner for petrol, and particularly if you are making a warm restart, it is quite possible however, and could be very unpleasant.
Petrol-fuelled engines always have their ignition timing set to before top dead center, unless they have variable ignition timing, which few mower engines have. So, the timing is a compromise between what would be needed for maximum power and fuel economy, versus what enables the engine to start without kicking back excessively against the starter.
In the case of Briggs side-valve engines in particular, very great attention has been paid to minimising the weight of the engine. As part of this process the size of the flywheel has also been minimised. The manual warns the operator against starting the engine without having it attached to whatever it is supposed to drive.
