Can an incorrect battery installation or reverse polarity in my Nissan Xterra truck cause damage to the knock sensor or the fuel sending unit

In a word, Yes. It can. Hooking a jumper cable up backward can and will cause all sorts of damage to electrical components. That is not to say it caused the fuel sending sending unit to short out and go out, but it seems too much of a coincidence that it happened right then. If this guy was a professional mechanic that installed the battery and/or jumped it, ask for any money you paid back, and for him to pay ANOTHER mechanic to repair all damage. If they refuse, there are several options to poke him with a sharp stick, so to speak, including but not limited to small claims court. If it comes to that be sure to bring along another mechanic to be a witness. But before you take it that far, try the Bureau Of Automotive repair in your area, the BBB, and get 100 of your friends to make signs and picket the place. Just kidding on that one.