I agree with you that Richard didn't give the best advice. Most of what you said was true. But, there were a few that I respectfully disagree with.
Hidden Fees
You mentioned hidden fees. Actually, this is a very common way that contractors scam their customers. It's so common that in real estate classes, it's taught as something you need to look out for. Contractors will deliberately write the scope of work so it excludes work they know is likely to occur. For example, in mold remediation, the scope of work will state cleaning the floors although the contractor knows that mold will likely be found beneath the floors as well.
Contractors will also make their bid/estimate low so they can procure the job. Later, the contractor will more than make up for the low bid by charging high rates for the work change order. (You can threaten to sue the contractor for bad faith but lawsuits are usually costly and stressful. It's rarely worth it.)
This "scam" works because it's almost impossible for the customer to look for another contractor when "hidden" work appears due to time constraints and costs. Even professional real estate developers will fall for this "scam". For example, the scope of work for the Empire State building didn't include the final floors. So, when construction reached the top, the developer was furious but was forced to pay exorbitant rates for the work to be completed.
One way to combat this is in the contract, to stipulate hourly rates for additional work. But, this will only lessen the cost of the scam; it won't eliminate it.
Upfront Deposit
I agree that it's sometimes normal. Contractors may not trust the customer to pay. And, deposits are actually mandatory for big jobs. However, this should be a red flag for smaller jobs because this is another very common way for scammers to take your money. Some contractors on hard times will take the money and not do the work. Some will even take the money and run. It shouldn't be the sole reason you don't hire a contractor but it should be something that makes you wary.
For smaller jobs, the deposit shouldn't exceed more than 1/3 of the cost of the job.
Finding the Best Plumber
This is actually an extremely difficult task because there is no good method for doing so. People recommend getting referrals but to be honest, this isn't a very good method because what does the person doing the recommendation know? And, frequently, the person will recommend someone they already have a relationship with. For example, it might be a friend.
Reviews are almost meaningless these days. We all know that most of the reviews on Google are not genuine.
To truly determine if the person is a good plumber is to test their technical knowledge and their integrity but how is an unknowledgeable customer supposed to do this?
One method that I use is to pretend to be stupid and ask the few questions that I already know the answer to. It's not the best method but it's similar to throwing a dart at a dartboard. There is a chance you'll hit something. So, you
might be able to assess their integrity, kindness, and expertise. For example, last week, I played dumb and the guy tried to upsize a new HVAC installation without doing any heat load calculations.
Unfortunately, scamming people is rampant in construction and almost every industry in our society. Lawyers, doctors, utility companies, retailers, employers, employees, etc. We all try to scam each other on a daily basis. Makes me sad.
"Some plumbing service providers provide the best installation services, some provide the best replacement and repair services, so you need to choose the one that best suits your specific needs."
TERRIBLE
A decent master plumber will be licensed in the area they provide services and are insured and do all aspects of plumbing
Would any normal person go to a car repair shop that specializes in spark plug replacement?
One call should do it all if the plumber is qualified
Asking for upfront deposit is normal if the materials such as a special order or color of fixtures are required and how large the job is.
I had deposits over $300,000 called "mobilization" funding to supply 10" cast iron and related fittings along with 8" diameter copper
Regarding "Hidden fees" unless the plumber has X ray vision how do they know what is behind the walls or under ground?
A video inspection is useless if the sewer, waste or storm lines have waste or filled with water as this will hide any defects
When working on older systems it is common to find lead waste and even some branch work that used lead pipe on the potable water system