During the combustion stage, How important is it to have the entire gas heated at the same temperature? And how can you go about getting the best result? Also do the kinetic energy of a gas change instantly going from low kinetic energy to very rapid when you introduce extreme heat? Even though it's only applied for a millisecond. Does the gas need to make direct contact like it does when it gets enveloped with the heat to change kinetic energy and cause as much pressure as needed for a piston to be forced down?
As method 5 mentioned it is important for gas engines and not as important for diesels.
In a gas engine in a car you can foul spark plugs, catalytic converters, and create carbon deposits not to mention lower performance. In a diesel you will have carbon deposits and on ones with catalytic converters you can fowl them.
you can ensure more fuel burns by adding more spark plugs per cylinder, creating hotter sparks, atomizing the fuel better, heating up the fuel charge before it enters the cylinder, increasing the compression, and making sure your air fuel mix is perfect.
the kinetic energy is not instant it's a wave more or less and this is why spark happens way ahead of TDC at higher RPMs because by the time the piston passes TDC the pressure from the burn will have finally caught up.
The last question you had is a little confusing but I will try to answer what I think you are saying. what happens is the heat energy from the reactions of the fuel and air increase the molecule's energy and make it zip around faster and some percentage of all these now moving really fast molecules hit the top of the piston and when enough of them hit the top of the piston it moves down. if there are too few molecules they may not hit the piston as explained here the worlds smallest steam engine