It is totally off topic but...
I don't agree with the statement that kettlebells are superior to dumbell when overhead pressing or performing a Turkish get-up, doesn't matter from whom this comes from.
I have pressed and performed Turkish get-ups with both for that matter.
The whole scapula story is all to be proven (good luck) and very arguable.
Dumbells did substitute kettlebells for a long time for a reason: they are more comfortable and practical.
Fashion, of which the fitness industry desperately needs in order to keep or improve profits, is a completely different ballgame.
A kettlebell is useful for light swings (since up to 40kg is light and I have yet to see a kettlebell heavier than 40kg), for pressing holding it upside down (preferably with another kettlebell or plate on top of it) and for juggling, everything else is better performed with dumbells, period.
For some unknown reason your post stopped at "...some sort of cable chest fly?" when I read it, therefore some of my thoughts about the rest of it.
....on exercises using parallel bars or gymnastic rings, I can tax my muscles at a seemingly 90%-100% flat out maximum effort level through to eccentric failure with these long sustained isometric muscle contractions....This can easily reach the relative intensity of barbell squats to failure.
Nope, maybe kind of a similar perceived effort (but in that case I doubt you ever really went to failure close to your potential in a squat), the physiological implications are quite different and not really comparable, it all comes down to the size and number of muscles heavily involved.
Regarding how heavy is heavy....well, from a training point of view heavy is when you go close to failure regardless the number of reps, in regard to Dr. Natale statements (of which a link to the full study or whatever would be appreciated) I suppose a near max effort is considered, something that is very near to failure below 5 reps.
Obviously your near to failure for 3 reps would likely be quite different from mine and even more so from that of an elite powerlifter, I suppose.
It's not clear what weight x intensity to target across which muscle groups for the best risk : reward ratio, particularly as strength develops.
It depends what you mean for reward, obviously the reward is the obtainment of your goal...therefore it depends by your goal...which, again might be not the same of mine or of an elite powerlifter.... or of a professional bodybuilder, for that matter.
It seems like higher reps and isolation exercises carry a much greater risk of injury / RSI -- although, maybe these are safer if performed correctly?
Higher reps do carry lower risk of injury provided the correct technique is employed, an herniated disk, for example, is very unlikely the result of a single heavy rep but the outcome of thousands performed with a wrong form.
Isolation exercises, or mono-articular exercises to be more accurate, do carry a greater risk of injury especially is performed poorly and/or at low reps, those aren't meant to show everybody how strong you are, generally speaking less than 10 reps on those do make little sense and are risky.