Valve cover isn't covered under powertrain. That's just the regular warranty, I believe.
If you're getting bad compression and your pistons rings are shot and you need a new engine, that's covered under it.
No misfires and that amount of oil in the spark plug well makes me think gasket issue.
EDIT: This is what is says about the powertrain warranty. It does mention gaskets in the engine section, so you may be good regardless. It's not very specific, so don't want to give you a definitive answer.
Extended warranty coverage periods are available for certain
vehicle parts and conditions. Specifically,
(1) Your vehicle's Powertrain components are covered for six years or
70,000 miles, whichever occurs first. The extended coverage applies to
the Engine: all internal lubricated parts, cylinder block, cylinder heads,
electrical fuel pump, electronic engine control unit, engine mounts,
flywheel, injection pump, manifold (exhaust and intake), manifold bolts,
oil pan, oil pump, seals and gaskets, thermostat, thermostat housing,
timing chain cover, timing chain (gears or belt),
turbocharger/supercharger unit, valve covers, water pump;
Transmission: all internal parts, clutch cover, seals and gask
