Before you change the plugs and O2 sensor, you might want to do the Seafoam clean-out. Some have said it fouls the plugs and O2 sensor so it is best to do it before you put the new ones in. Search "Seafoam" and you'll find tons of posts telling you how to do it. BTW, I got 31mpg at 70 mph last time I took a trip and checked it. Summertime, not windy, relatively flat.

Yes.