Completely different car, but i had really bad fuel economy on a mini (2-3x), thought it was fuel leaking.
It turned out to be the air filter, now although newer cars have fuel injection, a restriction of air into the engine will still cause over fuelling, so it might be worth checking the air filter, as its cheap to replace.
Also, before replacing expensive parts, I would make sure the important bits are working properly. In particular I would clean all spark plugs, leads, distributor cap, and then clean out the throttle body using spray, and then use a decent fuel cleaner in your tank, an easy and brilliant one is red line s-1 or s-2 which will clean everything including injectors.
By the way the o2 sensor works in an odd way, it registers oxygen, not fuel, so if you get unburnt fuel going past it it will read high oxygen regardless, and assume the engine is UNDERFUELLING, hence adding more fuel and increasing the issue.
The kangarooing is a difficult one, i had a friends car doing this, and the throttle body was filthy inside, and with an o2 sensor reading, his engine bay was oily, and the battery had a loose wire, which wasnt a concern to him!
But as i say start with the basics first, as ive said, and then things like vacuum hoses and look really closely, one of mine went last week, and this is one ive checked lots before, and didnt notice as the break was under the metal securing clip.
