#2 misfire, changing plugs and have oil in #2 cylinder. Could that be head gasket?

Your just polishing nails with that diagnostics. Did you check your oil lines and oil pump? . Could be the reason the heads blew in the first place. Have you tried rotating the pistons with the nut on the pully? That should give you enough information to figure out what the problem is basing on how it feels(throw some oil on it good beforehand). And it wouldnt hurt to change the fuel pump either.

Sounds like he has it sorted. Get it thoroughly checked over for bad leads, poor compression on one or more cylinders, valve or valve gear damaged. Its 15 years old, has it been regularly serviced? What mileage has it done? Is there oil in the water or water in the oil leading to the head gasket leaking. Something which happens after running without proper coolant which rots the gasket.

Before you listen to friends about the head gasket have a compression test done to confirm the fact run fault codes again and fix all the systems revealed . Check for vacuum leaks from hoses to the vents and brakes.

Take it to the dealership if it's under 60k miles. If it's sucking oil, then it sounds like you might have a bad head gasket, which is covered under the warranty. There's been a lot of hinky issues with the 2009 Sentra and #3 cylinder misfires, but apparently they don't want to do a recall on them. It's almost as if they're betting on the problem developing after the warranty expires.

head gasket blown or warped heador cracked head on engine

Likely a head gasket problem, between the water jacket and the one cylinder. Look for bubbles in the radiator with the radiator cap off before it heats up. Or run compression check on that cylinder. See if it holds the pressure after you stop turning over the engine.

Well could be a cracked cylinder or cracked piston . First i would check the compression on each cylinder. If there is more than 15% difference there is an internal engine problem. The cylinder head would have to be removed and checked. and check the cylinders for cracks and bad pistons

you have a blown head gasket,or drove through a puddle and **** water in the engine,ethier case very bad

Sound like an air / vacuum leak to me.
Re-check for an air leak on #2 cylinder inlet manifold.
Use a propane torch (unlit, of course) to flood propane gas around inlet manifold/head flange & gasket area.
If there is any change in idle speed during this test you have a vacuum leak.

Does it run bad? Both cylinders are on the same ignition coil.