When coverage is difficult to estimate, add more rather than less. You can always pour the leftover back into cans. For large jobs, use the bucket and a roller screen rather than a roller tray. It’s much faster to load your roller with the screen than to use a roller pan. Simply dunk the roller into the paint bucket, then roll it along the screen until it stops dripping.
Not all people live where they can hire a painting contractor, like you describe. People who live in small towns can only hire painters who have a very small business, and do two or three paint jobs per week. In this case, you do have to be very careful, when you hire a painter, as we have several, in our area, who are out to make a fast buck anyway they can.
For interior paint I prefer the semi-gloss, provides a very subtle sheen and is super easy to clean -no flaking, chipping, etc. I've painted a lot, and found that Sherwin Williams Ovation paint has been the easiest to use, and provides the best coverage. I even painted my ceilings with it and WOW! I was done it no time at all, with perfect coverage.
Consider purchasing supplies personally to save money. Ask the painter for a bid that separates labor and materials. Then explain that you'll purchase the materials and ask for a list of exactly what will be needed to complete the job. Caulking, for example, is an extra supply commonly used to fill any cracks or damaged areas in your walls -- and one that might be overlooked in an incomplete list.
Second coats on similar colors are almost never recogicnized as being needed until the coat is applied and has dried. ONLY THEN WILL YOU SEE WHETHER IT NEEDS A SECOND COAT or not. Yes, painters can use a cheaper paint then what you paid for. That is solved by getting your own which, I would charge extra for because I will always have to go get more, or add second coat because home owner tried to skimp on paint, or they got the wrong color etc...
Painting your house is one of those maintenance projects that most homeowners undertake at some point, whether it's the outside or the inside. With the exception of hardened DIY-types (you know who you are!), just about all homeowners will hire painters at some point—whether to prepare their home for moving in, or for a sale, or perhaps to kick off a remodel with a new color scheme.
You are right on with this - why do people leave switch plates on when it's just so easy to take them off? Another thing that happened to us - we had the popcorn ceiling taken off and the ceiling painted white. When the job was done and I later went to change out all the fixtures/fans, they had left every fixture in place, so there was a large patch of popcorn and unpainted ceiling left behind - it just didn't dawn on me to specify that they take those down before scraping and painting. It was kind of a mess.