It will make it much less likely, but won't guarantee it. I know vehicles that have started to roll in gear. That's why there's also the recommendation to turn the wheels to the kerb as well.
Don't all automatics have a "park" position on the selector, which should always be used when leaving the car? I don't think I've ever driven one without.
Stop-start
If you are unlucky enough to need a flywheel, starter & a battery replaced at the same time you’re looking at a bill between £1500 & £2,000 (maybe more) depending on the car.
Holding the clutch pedal down as you describe, there is a bearing, the clutch release bearing, it will shorten the life of the bearing possibly triggering a need for premature replacement, expensive as the labour charge will be about the same as a full clutchplate repair
Japanese Honda introduced stop-start engines more than 20 years ago, then 15 years later the European car makers joined the game, the European systems are weak copies of the Japanese system. The Japanese used a separate direct-drive to the engine crankshaft brushless motor without any gears or starter ring to wear out in use, the Europeans simply adapted (beefed up) the existing conventional starter motor / flywheel ring-gear system.
On a car with a manual transmission, a "shunt" while parked, is low-risk for the gearbox, just think of the loads the gearbox routinely accepts when accelerating hard or climbing steep hills.
My Father managed to break a handbrake cable on his Ford Anglia, the situation, parking on a steep hill he applied the handbrake, then for "good measure" gave the lever an extra pull, the steel cable parted company at the clevis at the drum brake end of the car.
With some cars, mine included, it has auto hold, so it will hold the brakes on when stopped (lights illuminated). When at lights, particularly in the dark, I will often flick the parking brake, but I feel many don’t!
My VW and the one prior require(d) the clutch to be depressed to start. The Mazda I had before didn’t but I tended to do it anyway after a couple of occasions where I started the car in gear and lurched forward
It depends entirely on what type of trips you do. Almost all my journeys are either a long M-way trip to my daughter's, or a 30 mile rural round-trip to my nearest town with only one traffic light when I get there.
If the hill is steep enough. But more slowly than if they are in neutral.
Those "some" are correct. FWIW I was once the deputy foreman in car dealer's workshop. The part that wears in that situation is the release bearing, and the wear is caused by the load on it, with the pressure provided by your foot ultimately. I have replaced quite a few in my time, they are not particularly robust.
I may be wrong, but I'm pretty certain that on my Auris, you need to apply the handbrake and be in neutral and release the clutch before the stop/start kicks in. Thus it very rarely only kicks in for a couple of seconds because I've anticipated the changing lights and have so not fulfilled all the criteria for it to kick in.
How does the 'stop-start' feature damage the car? Mine is excellent and very efficient, but as it's a hybrid car and most of my journeys are on electric power it doesn't get used very often. It also has a 'hold' function for the brakes when coming to a halt, which is useful. Prior to my current car I had manuals, but would never leave it in gear when parked and would be in neutral at stops unless I knew them to be be very brief.
That happened driving my dad's Austin Cambridge when I was 18. My current Nissan Pulsar will safely park on a steeper slope on the handbrake
