Thanks for the response and info bertsmobile1. It does sound like the problem could be bad fuel or a fuel supply problem but I'm pretty sure this is not the case here. None the less, I did suspect bad fuel or a fuel supply problem early on in my initial diagnoses nearly 2 years ago. The first major effort I took concerning this problem was to remove and empty the fuel tank, clean the tank, remove and flush out the fuel line all the way to the carburetor, changed the filter, and refilled with fresh fuel. As with all other attempts to remedy this "intermittent" start problem, I've had a couple of times when I thought the problem was solved only to find out the next time (a week or so later) that I'm right back where I started with the mower failing to start.
A little more background... I own 3 boats with 140 to150 HP outboards, and often run offshore up to 50 miles from land. I'm anal about using fresh, clean fuel and so I'm aware of the many of the common problems related to fuel and fuel systems that can leave one stranded far out to sea in a life threatening situation. Here at home, aside from my Hustler zero turn, I own and use 2 push mowers, 3 chain saws, a log splitter, two garden tillers, a yard vacuum, two weed eaters, and a hedge trimmer and have maintained these and many other similar types of gasoline powered lawn and yard equipment over the past 50 or so years. Also note that I purposely keep only enough fuel to last 30 or so days so I'm constantly replenishing my fuel supply on a regular basis. No other of my gasoline engines have experienced fuel related issues over this same period of time, or for many, many years for that matter, and all my yard and garden equipment uses the same source of fuel. I try to service and maintain all my small engine equipment myself and usually I'm able to diagnose and repair just about all the minor issues a fellow might come up against trying to do so but this problem has been long lasting and so far has gotten me stumped.
Having said all that, I'm going to run down each point you've made tomorrow (or the next time I'm unable to start the mower on the first attempt) and see if I've missed something that you have mentioned that might help me solve this problem.