1+2 : The very high temperatures at the exhaust valves. This stresses the valve material to a point where it is starting to soften and is lengthened by valve spring preload. Same high temperatures lead to small deformations at the cylinder head (exhaust valve side is much hotter than inlet valve side) so the valve seats get a little oval and high gas is blown by, causing even more deformation.
Valve seats are additionally loaded by lead deposits and carbon buildup due to rich mixture.
4+5 : dirty fuel. Refill just at modern stations with high usage rates, they should have the cleanest tanks. Fuel filters in airplanes are different design as for cars, where much much finer filters are used. But theese fine filters tend to block completelzy if contaminated with to much water (wich is normally in sollution on cars due to the high ethanol content of mogas, but is dangerous is your fuel gets very cool at high altitude and ice cristals might build up at the filter)