Most of the curveball argument I think relates to a converging of what I am becoming convinced are a few ingrediants that combine to "push" the young arm over the "verge" from healthy recuprative arm, to a "sick" arm that is inhibited in it's ability to "heal". I don't have a orthapedic degree, or physiology related degree, but what I have is experience over years. I have seen a few pitchers and kids over a solid 35 years of truely being around and attempting to learn about it.
I have seen the advent of lowering the mound and the strike zone shrink by half. Iver seen the common method of pitch delivery change, in some ways significantly. So for what ever that may be worth......
1st is lack of knowledge. Throwing from a mechanically flawed platform to me is the first "key" , it is not only an ingrediant it is an accellerator. It would seem logical (Disclaimer on science to back this up), that if you throw in a harmful way, to continue to do so will speed the onset of negative ramifications (Speed injury).
2nd is over use. I've never heard anyone that claims knowledge or expertise on the subject, suggest the insane amount of innings that kids today pile up (Travel ball, spring, fall AllStars for 8 year olds or 7 or 6). Children are not built for repetitive stress in that way (Again with a disclaimer) and I don't think a thoughtful person needs a degree to expect that this assumption is correct. To do so would make you take the opposite or neutral point (Either it's detrimental, It isn't detrimental or it has no effect) which IMO is logic a "reasonable" "rational" person could find no "reasonable" argument for. When mixed with the first "key" factor I would guess that injury potential has been increased.
3rd is the failure of the kid to get proper rest between outings and to take proper periods of time to "rest" up. What I've heard described at the college level as "Active rest" and what I like to term for pre-pubescent children as "being kids and having fun" (Disclaimer for science). Well combined with one or two I think a reasonable person stops considering the possibility of injury into the speculation as to severity and treatment of whichever problem manifests first.
Ian may have a plan in place, and he may be very careful and conservative in his approach. Does that assure his sons or kids he coaches no injury? As a student of history and success, I can see that pitchers have thrown curves and never sustained any injury, some have barely made MLB and had to go under the knife, some didn't make it out of puberty before they got the scar. Throwing one type of pitch unless thrown for a prolonged period with no allowence for recuperation, wrong mechanically, does not nessecitate injury but each of those factors mentioned speed the onset.
To sum it up, Ian's boys or my sons won't become crippled for throwing a curve, but if each wants to throw it into the future they have a responsibility to themselves and their future to do it "right" or they will suffer negative consequences.
One other thing I do disagree with you directly on is the amount you need to practice throwing it to be effective with it. My son has what is termed a "plus" curve, I've heard it described by others as one of the best in Florida HS, he learned it by simply standing a couple of feet away and learning what the arm motion was by flipping it back and forth with me (Fastball motion until out in front, then down). We would work it in to our nearly daily playing catch until, on flat ground he could effortlessly toss it to a spot, this was done at 50-60% just tossing and messing. Never on a mound. We have never workied the amount of reps you said it would entail to perfect. We never made it a "big" deal it was just another part of warming up he didn't do "bullpens" of up to 60 pitches until Jr High and his curves in that totaled maybe 7/8. I decided instead of travel ball he would spend the hot parts of summer after AllStars going to camps at UNF (4 weeks, actually less than daycare would have cost), where he got more understanding of the responsibility to his body, he also got to swim and play these really great rubberball games in a softball field for fun, hanging out around college level players and learning from them, instead of grinding through the summer pitching back to backs at tourneys. With the contacts I made there I was able to get our association to do some clinics with UNF, which was great for the kids, but much "better" for the coaches who learned that they needed to learn and also started to understand mechanics. Anyway I don't think he has ever thrown 500 pitches practicing any of his pitches.......maybe in 12 months, until he was a sophmore.
Now as a Junior he has the "vitality" to last year have 10 HS level CG's. (JV and varsity combined including summer hs ball).
Sorry so long