I suspect you have a case of broken rings actually. if the valve seals are reasonable and not hard (compare them with the new ones) and the valves not overly loose you certainly need to pull that piston and check it out.
But yes,that will cause preignition and a lot of other things such as the oil leaks which will be exacerbated by the blowby from the sick cylinder as crankcase pressure will overcome the seals and the idle air suction system.