Nice post. At last a thread worth replying to in the WA section! Yes, there's lots of cowboys out there!
I think every industry/specialist field is like this. Take IT for example, there's generally a lot more than 2 ways to accomplish the same thing. Option A and Option B.
The correct way is often determined by experience in the end, ie Shop A might do something a certain way and possibly a better way due to later down the track it can cause Issue A. Shop B might not know about Issue A and continue down it's only line of experience to achieve the same outcome.
Ditto with software development - Although add another 9384938 ways to do things and you're pretty much on the money.
It comes down to the end fact, either you learn it yourself and make your own conclusions and methods to achieve what you want (As you have expressed above). OR place your trust in a Shop with the years of experience that you cannot achieve in your short timeframe.
Now, I don't claim to be an expert in cars. Although I do not know a lot more than the average bear when it comes to diagnosing/problem solving and/or "the way things work" I like to investigate EVERYTHING myself before taking it to a workshop to do (some previous dodgy-jobs I've done on my car I've had to re-do because of this) but hey. It's all fun in the end.
And regarding Point #4 - Customer service. I've found this pretty much doesn't even EXIST with the motor vehicle industry except for a few shops. Maybe years of know-it-all tyre-kickers and whingers have soured thier willingness to help the average Joe. A quick courtesy call, a technical explanation of an issue, FIXED pricing, accurate quotation, accurate timeframe estimates for jobs, etc etc (I'll stop now)
Good luck in your pursuit for knowledge by the way