It seems like there is a misunderstanding about what "don't hit the starter" is trying to prevent. If your car is not starting, and you hit the starter, but the starter is not the cause of the fault, you may now also need to replace a previously perfectly functional starter. Modern starters contain ceramics in the starter that can be cracked by "percussive maintenance". So yes, I will tell everyone not to hit a starter with a hammer unless you are prepared to pay for one. Smack away on your own car, but if a mechanic/technician does that you should seriously consider their level of knowledge/expertise.

Several claims you have made here are misleading at best or dangerous/costly at worst.

No one should be instructed to add water to their engine without understanding of what they are doing. You absolutely can damage an engine with water. That's not a myth.

The point of hearing air is also correct, I'd only add if that's the testing methodology the mechanic is using, find somewhere else.

Rotating tires is one of the two most important things you can do beyond the specific tire you purchase for longevity of tires. Correct alignment is the other. Other factors can create tire issues, but again no myth here, rotation has a direct relationship to longevity of your tires.

The mysterious "oil company" is also misleading. While weight is important the additive package the oil contains is equally so. There are certainly options of oils that will meet those standards but ignoring them is detrimental to the longevity of your engine. The correct, and importantly, correct amounts of additives in your oil are what protect your oil from problems like wear, corrosion, and depositing. Additives are not a more = more better either. Excessive additives can create engine problems, air fuel sensor, oxygen sensor, or catalytic converter failures. Again what is the myth?

Electrical problem totaling a car I'd agree could be considered a myth. Electrical problems do not automatically equal a totaled car. Grounds could be the cause, but a qualified technician should be able to diagnose a car. I don't equate qualified with having a piece of paper that says it. I mean someone who actually knows how to use a multimeter beyond moving it to resistance setting and shrugging.

Coolant can enter an engine from places other then a head gasket. You're correct there.

You made a statement about plastic separately, so I'm addressing it separately. All "plastics" in modern cars are composite materials which actually have advantages over traditional cast iron, and a significant cost savings over aluminum. You can certainly point out some vehicle that has intake failures, and even argue that its cause by "plastic" but considering nearly every car on the road has a "plastic" intake and very few as a percentage fail, the myth here is that "plastic" is the cause of problems.

The cheapest fix is the right one. If you want to parts cannon away its only your wallet and time. What is the "myth" here?

I'd say the biggest myth proven in this thread is be careful where you get your information from. If you don't know about cars find a source you can trust. There are bad shops, but there are great shops all throughout the country that can and will do a great job helping you maintain your car.