Your lawnmower never would have been safe in my hands, or at my hands I'm afraid. When I was a kid my sister and I decided to mow the lawn with this old push mower and we accidentally ran over a flip-flop in the grass, slicing it to smithereens. My mom to this day says "That could have been somebody's foot!"
Well, if you worked both sides equally then that is a good thing but I was wondering if there was any imbalence in the blade the higher speeds would be very hard on the bearings. It sounds like something I would have liked to do myself given the grass height on a few occations around the house..
I just love the story of the lawnmower redo! It reminded me of those folks how race lawn tractors after souping them up (looks like a lot of fun). I was wondering if you had any issues with the blade balance after sharping it up? Especially with the higher speeds you were running. Or did your father tell you about balancing the blade (using the typical trick of suspending it on a nail through the center hole)?
A friend of my son had a lawn mowing business a few years back (before college) and he earned a LOT of money. He never even considered going to work for someone else as he had all the customers he could handle and made really good money. As with your experience, he was very conscientious and worked hard to keep/make customers happy.