Craftsman 7.25 Platinum Mower Won't Start?

My mower won't start and here's the situation and what I've tried so far. It has been running this spring even with a tankful of last summer's gas. But I wanted the blade sharpened and a service (spark plug and air filter check) so I took it to a free community fixit place and watched as the guy removed my air filter which was in pretty good shape with lots of white and just a bit of debris, but didn't see the filter when he replaced it.

He checked the spark plug and sprayed something in the carb and it started right up. I guess he sharpened the blade. Anyway, I came home and the mower ran for about 10 minutes and then stopped running! It wouldn't even turn over. I pulled the spark plug and I don't think it's the same because it's not the part number the manufacture suggested. I purchased a new spark plug. Still won't turn over.

Then I checked the air filter and the one in there now is about 5 years old and filthy dirty. I am not sure why that guy felt he needed to take my good one, but whatever. So I purchased a new air filter, but still nothing. So, is it possible that the mower ran on last summer's gas for a little over a tank then quit?

Also, I pulled the new spark plug and it smells like gas and is wet, does this mean that when the guy laid the mower on it's side to work on the blade that the gas and oil are now in the engine and it so, am I doomed? If not, should I remove the gas and start with fresh - obviously yes, but at this point will it matter? Thank for your help!