Dr Pepper > I actually disagree with you on the part about a non-professional being responsible for damage caused when they had permission to work on something.
I probably missed out some important caveats. You can look at, for example
YD 336:6 – liability of doctors – experts v not
liability for advice – expert/not, paid/not – Nesivos Mishpat (306:14) CM 306:6-7
indirect loss CM 386:3