Maybe I wasn't too clear on my explanation.
It is possible that the misfire ( caused by some ignition problem ), is responsible for the fuel trim code.
At 98 k miles, an O2 sensor would not be a total waste of money.
As far as special tools required, you can buy ( at any automotive jobber ) an O2 sensor wrench.
It is a deep socket ( 21 mm ) with a slot cut into the side, where the wire exits, before you attach your ratchet.
Should be able to get this for @ $ 10.00.
Btw., the hex portion on ALL O2 sensors is the same.
So, you're not buying a very special, or make-specific tool.
