What you really want (as far as I can tell) is your car working and to be done with John. So why not take up the offer here and get it fixed. I agree, you don't need any more stings from the scopion and I'm not sure the confidence is there in John, justified or not, that he can do the work that needs to be done. If he can, why didn't he just do it the first time?
I'd stay focused on what you really want, which is a reliable, properly sorted car you can enjoy. And I think your best shot at getting that is by embracing the collective experience and skill of the group.
One man with a passion bests one hundred with a job. Having a passionate team with experience offering to help you is a no brainer for me.
PS As far as I can tell this is about the point in the thread where someone should start posting "garage art" of scantily clad women. Far be it from me to stop anyone.