I agree with most of what you've said, but this bit causes me some concern. While you cannot account for previous work by others or for damage caused by the user you CAN and SHOULD be prepared to warranty your own work. If you open up a gun and can see that there's been some previous bodgery that would likely affect the performance of the work you've been asked to do then I for one would expect to be contacted at that point BEFORE you carry out any further work and informed of the potential issue and asked if I wanted to continue with the work asked for. At that point I would then think it perfectly reasonable for you to turn to me and say "look, I've warned you that the parts/work previously done may cause an issue so while I stand by my own work it may still fail due to the previously mentioned problems". If I wasn't warned about it prior to getting the gun back and indeed paying for it then I'd be pretty pissed off if it broke and then got the standard "yeah well...it's them other bits what someone else fitted innit" response.
Unfortunately the latter case of the idiot customer that deliberately abuses the gun and then tries to pin it on your work is all too common. If there's an obvious pointer to user based issues (in the software engineering world we call it a PEBKAC issue - Problem Exists Between Keyboard And Chair) the you could justifiably point to that evidence and say sorry, not my fault but I'll fix it for you at my standard rates. Of course if no such evidence exists even if you KNOW deep down in your heart of hearts that there is no way this could possibly have been caused by anything OTHER than a fuckwit being let loose with a shiny toy then I'm afraid you just have to swallow your professional pride and fix it anyway or tell him to get lost and risk the internet based wrath of a pissed of customer. Who is ALWAYS right, like it or not.