I share your frustration with repair shops and do not trust them a bit. However they can not be avoided, and the only way to alleviate that frustration and to get things done to both sides satisfaction is to learn to work with them, rather than approach them with the mindset as established above.
Through my work, I am often on the receiving end of having to deal with difficult to diagnose and fix machinery issues. When a problem is difficult to diagnose, and often there may be overlaying issues that compound each other in mysterious ways, it is next to impossible for a repair person to diagnose it right unless the technician is very experienced and has encountered the same problem multiple times. Tesla technicians are not experienced as the car itself is young.
In my experience, the fastest and most effective way to solve these difficult to diagnose freaky problems are by harvesting the clues on the users side. Simply relying on the technical expertise on the repair side leads to ineffectiveness and frustration. User has all the clues on the problem occurrence but may lack the technical knowledge to put it all together, hence a comprehensive dialogue between the user and the repair side is the fastest way to get to the bottom of the issue or multiple overlapping issues.
In your case, it seems that you already have a solution to your problem. I am curious, what is stopping you from suggesting your solution to a friendly technician? They may be doing their best and putting all their heart in their job, but that is not enough, as evidenced by your customer experience. They simply may not know what to do to fix your problem, hence they seem to be doing something that they do know, as you suggested in you post.
I really do hope that you get your problem fixed, as it is disappointing otherwise.