Classical dressage here, this is totally one of my pet peeves. No flatwork isn’t dressage.
Back to the questions:
Balance is balance.
I want my dressage horse a bit more on the vertical with the nose, still out though…
but you can (and IMHO should) ride HJ horses with an elevated shoulder and freely moving back. Since its about the back not the head, the head can go where I need it to go. I ride up/down, more nose out, etc. (Just never “deep”).
My horse is/was a good green jumper because he has excellent balance and straightness, but he did get a bit rattled when he wasn’t supported as much in the aids.
I have to be very careful how he is introduced to gymnastics so he still feels we have a conversation. He is a worrier naturally, and very sensitive.
I don’t “get” the resistance to dressage, its so good for all horses. Maybe its lack of education on why we ask for balance, rhythm, straightness, and collection.
Never understood how you can go around a course with a stiff braced horse, pulling and going on the forehand. No thanks’
But no, the average 2-3" horse probably won’t need to be collected, but the joints of the front legs will last longer if not getting pounded on the forehand.
USE the movements, they are going to only help!