Another question to ask of technicians out there is this. What if, before
you begin, the customer says, "Ok, I'll pay you $85 an hour to diagnose and
fix the (whatever), but only on the condition that you're ultimately
successful." Would you go for it? What if the number of hours or cost had a
cap on it?
This approach seems fair to the customer, but I bet most car mechs would
rather do brake jobs than a deal like this.

Yes, a grey area. The only thing I can think of is to specify the conditions
as much as possible in advance--but then you sometimes get caught, as Joy
did, in a place where you're dependent on the mechanic, and don't have much
room to drive a hard bargain.