Hi Ben,
Given the age and mileage it's possible the plugs and coils both need replacing, unless you know they've been done recently. Your diagnostic tool should narrow the problem down. There's an issue with some 2.1's where water collects in the recess around the rearward coils (cylinders 1,3&5) and can get in and corrode the plugs. It's a DIY fix if you're patient. The airbox needs to come off, the manifolds don't need to be moved, but it helps to have small hands! I had all six changed on mine in January after intermittent mis-fires and it runs really sweetly now.
As for MPG on a 2.1, mines an auto and I get about 22mpg around town, but up to 35mpg on a good run. I run it on standard BP unleaded and put an additive in every 2 or 3 tankfulls, which is about 4 times a year for me.
Hope this is helpful,
Cheers,
Ian