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1) Lack of back pressure will hurt FE. I've seen it multiple times with my own vehicles as well as with others. Fix it and it goes back up. Leaks and breaks kill low rpm torque.
2) If you want to think about reusing the CAT you can try looking through it. See if there is any physical damage or clogging. Replacing it at that mileage is probably a good thing, but chopping one out and clamping a new one in isn't a big deal either, and if you get say 15k more miles thats another year or so with the truck.
3) Moving the muffler will change how things sound, thats about it. Emissions wise it doesn't matter, functionality wise either. It'll just change the tone.
Smaller piping will increase lower rpm torque at the expense of high rpm torque (less peak horsepower) and also has the potential to increase FE. I wouldn't even think about going larger.
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