You say you’re looking for a young horse. Not sure if that means before or after it’s been backed and 90 days put on it, but understand a lot can and will change depending upon the kind of training rides horse receives. In other words, the youngsters you’re watching free jump now can either be improved or ruined by training. Which goes without saying.
I think it’s impossible to differentiate unconventional from poor style. Technically, they’re both the same. And lets’ face it, an ‘unconventional’ style is NOT going to do well in the hunters. Period. Full stop. Jumpers need to jump clean. Correct style helps achieve that but your unconventional horse won’t be shot down so to speak in the Jumpers. Eq horses need to be more even, flatter jumpers so as to not dislodge their equitation mannequins perched upon their backs.
I could spend my time trying to find resources for you with respect to your specific questions but I"m going to instead let you do the hunt… Google terms like ‘jump bascule’ for a start.
At its most basic description, you want a horse who’s jump effort is even across the obstacle. That’s bascule. You want the horse’s body to create an arch as it were from take off to landing and lightly cantering away. Horse shouldn’t spring up vertically and land in a heap.
Horse should be free in his shoulders (which is why so many dislike ‘straight’ shoulders), and inso being, be able to move his forelegs well up in front of him. His forelegs need to be well above the horizontal at the height of his effort, knees tucked together tightly under his chin. Neck and chin stretched down as horse starts to descend in his bascule. His back should be round longitudinally and straight laterally. No twisting is considered ‘poor form’ or ‘unconventional’.
Horse should have the intuition to know to depart the ground well enough back from the obstacle to allow himself room to get his forearms up/ out in front of him. Horses that habitually get in too close wrap jumps with their knees. Horses that leave wa-a-ay too soon will catch hind legs.
Again, all of this can be improved, likely as much as 75% or more, with good rides and training. I’d be more concerned with conformation limiting good jumping rather than just seeing what a few jumps in a shoot on video look like.
I like short backed, slope shouldered, deep & straight hocked horses. With this as a foundation you can’t go too wrong.