For every 'expert' that says one thing, there can be found another 'expert' who says the opposite.
Perhaps, but smoking causes cancer pretty much unarguably these days - but if an 18yo gets mild cancer it's highly unlikely that it would be attributed to the fact she smoked a few cigerettes behind the bike sheds or even if she'd smoked for a few years.
The reason smoking had decades of experts arguing about it, aside from the large amount of money thrown at one side that was very interested in it not being the cause, is because unlike something like, say, drinking bleach where it's difficult to argue, the effects are pretty long-term.
"Smoking causes cancer" doesn't mean you'll get cancer a week after starting, nor even 5 years. I think even the old wives that have told me for years that cracking my fingers means I'll get arthritis weren't talking about getting it within 10 minutes of them telling me
I'd accept that cracking the knuckles might injure your hands though - but it's a bit of a truism - indeed, contrary to popular things you've said and (c) before, I think you can injure your hands playing piano correctly or doing anything with your hands correctly - it's less likely than if you do it incorrectly but it's a possibility. So yeah, you can injure yourself in a variety of ways - OTOH, specific injuries can be attributed to specific bad habits - those injuries are far less likely to occur unless you are doing something over and over that leads to it.
I could see someone going around old people's homes looking for a correlation between knuckle-crackers and arthritis sufferers either will or won't find one [although aiui, when they have done that, they haven't found one]
But the idea that cracking your knuckles has a short-term effect like inducing mild arthritus in kids seems very dubious.
Topic: Piano Hands are Valuable! How pros treat their hands!