In the 70's I worked as a mechanic on commission for a while myself and I saw what was going on.
The bold part is all I need to see. Yes, in the 70's auto repair was more crooked than legitimate. I started in the late 70's and saw the crooks getting weeded out by the dozen. Laws were changed old parts required returning, etc, the upsell to a fake wash as you described was chased out of the business for the most part. By the early/mid 80's the major crooks were gone. Yes, there are still some, but they don't last long.
The leaving out time consuming steps.... Yup, time consuming, not quality affecting. I'll give two examples.
Mid 80's Taurus heater core. Whoever wrote the shop manual did it by computer not actual work. You have to remove the whole dash panel. The directions have you removing all the attaching screws at the top. Then you drop the steering column and other under neath parts that stay in the car before pulling out the whole dash. The LAST step after dropping everything from the glass to the floor of the vehicle? Remove the five or six screws that attach the bottom of the dash to the firewall. Screws you cannot even see or touch at this point. To remove them requires two more guys to hold up the dash while you crawl under to get them out. If you pull these screws first, you can do it in less than a minute rather than 45 minutes with 3 guys.
A recall for fuel system issues. You have to work on top and under the truck in multiple places. Following the directions you run the vehicle up and down on the hoist about 6 times in each direction. If you plan ahead and do all of the upper work, then the lower work you take an 8.5 hours job down to about 5. My best guess as to why it was written this way is each subsystem repair was written by the specialist for that subsystem. Then another guy compiled all of the repairs into one single list of directions, not looking at how each was done.
The other side of flat rate no one seems to know, or care, about is if it rakes longer. If the book rate is 3.6 hours and if something happens that it takes the mechanic 6 hours to complete guess what? He gets 3.6 hours pay for 6 hours labor.
Another part nobody knows about is backflags. GG and I have hinted at these. (This may have been part of the 80's clean up too) You "fix" a car and it comes back not fixed (comeback) you generally get to work on it for free. If the customer gets a refund because he took it somewhere else you get "backflagged". That means you get a negative flat rate fee put on paycheck. IOW you get a deduction for that work you performed poorly. There are many reason for backflags, all related to work performed poorly, or incorrectly, and not just tied to refunds. That was just an easy way to describe them.
Knowledge and experience can make flat rate profitable for the mechanic. Not through "cheating" but experience and skill. It can also be the worst way to get paid. There are two ways off the top of my head. Poor, or basic skills, you don't get the job done equal or under the book time. That will correct over time, one way or another. Or, and this one really stinks. For whatever reason, Holidays, vacations, poor economy, local big employer closes, etc... very few customers come in the shop. Now you have ten mechanics all facing a Friday with10-15 hours pay coming. (not 40+) Each wants to feed his family and each wants every job that comes in the door, but there just isn't enough work that week, or month. Times can get lean at no fault of your own.