Poor MPG and P0420

Hoping to get some direction. Here's the deal:

03 Forester XS, 218,XXX miles. I've come to realize that my fuel economy isn't doing so hot. 18-19 mpg at best, mostly highway miles.

P0420 a couple weeks ago. Local mechanic diagnosed it as needing a new cat converter. I didn't think to track my MPGs until this came up, turns out gas mileage was suffering before P0420, and seems to be getting worse.

Front o2 sensor and plugs were replaced at 178,XXX (when I purchased it). To top it all off, went through old service records, and can't find any evidence of timing belt being changed.

I'm having to add engine oil every 1-1.5k miles.

In the summer, I had a wire to the alternator fry (fixed). Since then I've had a few stalls (while idling). Tightening down the connection of the previously fried wire to the alternator has done the trick, but thought it was worth mentioning if it could somehow be related to these other issues.

Thoughts,
CR