It can't do what is known as kickback unless it has fuel and spark. Kickback is the engine firing far enough before TDC for the crankshaft to reverse direction violently, propelled by combustion in the chamber. It is technically possible to get kickback from a very hot engine without ignition, due to preignition, but you seem to be saying this engine is cold.
If a cold engine with no fuel and no spark resists going past top dead center and reverses direction, either it is mechanically jammed or the compression pressure exceeds the strength and momentum of your arm. It sounds as if the decompressor may not be functioning, even though you say the decompressor itself has been tested and is OK. The next thing to check is the hose running to it. I suppose it might also be possible that the decompressor port in the combustion chamber is carboned up, or you have a Powerplus head.
