2005 Persistent Misfires (P0300, 301...)

2005 Sentra GXE, 85000 miles.
Got a P0300. Replaced the spark plugs.
Came back after a few days. Replaced the PCV.
Performed the FSM procedure to check spark - all coils tested good.
Came back after a few days. Replaced upstream O2 sensor.
Came back after a few days. This time it was P0301 and P0302.
Car failed to start. Cranked - seemed like no gas. Took it to a mechanic. Engine was flooded. He cleaned plugs and it started. We talked and agreed to replace the Camshaft sensor and Crankshaft Sensor.
Came back after a few days - P0300 and P0303. Very frustrated.
Fuel pump was replaced two years ago. That with flooding seems to indicate fuel is NOT the problem.

Noticed that the SES light seems to show up a few minutes after starting. Car idles a little rough after first start. After warming up, seems to run OK. Drove it on a 200+ mile trip on the highway with no issues.

Any ideas?

I do have two questions.

1. What provides the signal to the coils? Is it the camshaft/crankshaft sensor or do they just monitor? Basically - what serves as the distributor?

2. Mechanic said he needed to change the oil after the flooded engine. Said gas can seep past the rings and get in the oil and damage the engine. I've never heard of that. In the old days, we didn't worry about a flooded engine. Is that really a problem?

thanks in advance. About at my wits end here.