I think the idea of getting a mechanic to work on your car after that mechanic's normal buisness hours may run into some problems, in a legal aspect. First, if you obtain a mechanic from an auto-shop (dealer's service dep't or not), will the shops' warranty still be applicable. I think not, b/c the owner of the auto-shop can say the job that was done not through company (auto-shop). So, if you did obtain a worthy mechanic, or just a mechanic, what happens should that mechanic screw something up - that may take more hours to correct than that particular mechanic was willing to work for, simply b/c he has a 9-5 job.
I think prudence dictates that if your going to obtain a service (not a product) from someone or something, one should get some kind of exit plan, should anything go wrong. The best way is to go to a shop, especially nation-wide type shop b/c they will have the resources to correct and refund any money at a timely fashion. More importantly they want repeat business. Or should I say they survive on repeat business. Also, a threat of exposure through the media will motivate any shop keeper to correct mistakes. One letter to the BBB or state attorney general's office will prompt shop owners to man-up to thier errors. An independant mechanic may simply not want to do that b/c again, he or she has a stable job already, and really is only seeking extra money, not to replace his/her job by taking on independant work. The mechanic working on your car is doing so on their spare time, it make take months before that mechanic has the right amount of time to correct the mistakes. True, that if you get a top mechanic more than likely he will do a good job (sorry he or she will do a good job), but top mechanics are human too. And they too can be prone to make mistakes, especially if they are having any kind of personal problems (divorce).
I would not tip a service advisor simply b/c the prices being charged are high enough, and if the mechanic is not getting paid enough, that really is the problem of the owner and lies with his consicous, not mine. And should the shop do a bad job, Well that is when i use the full force of the legal system, which I am an active player with. They mess with my car, I'll mess with thier legal disposition.