My experience with tearing out hedge trees with a track loader or dozer is that a significant amount of the root mass left will manage to sprout hedge bushes. We had 7 acres of hedge treees cleared by a track loader. The operator got all the stumps, but we were still spraying fresh growth hedge bushes 3 years later. Cutting and killing them is the way to go.
I prefer shears that hold on to the trees instead of the pushbar/freefall models. It doesn't take long to make a mess in thick trees if you can't control where the trees go. Even with rubber tracks you don't really want to be driving over cut hedge trees. A bonus to the grapple style shears, it's easy to build a burn pile or bunch trees without having to switch attachment constantly.
As far as what loader will do well in rocks and steep ground, anything that doesn't have an ASV undercarriage would be a good start.
Do you think that you could get close enough, and that a shear would have a prayer of getting hedge trees? I've never run a shear taking out trees, but I have a hard time seeing one being able to take out a hedge that is any bigger than 6" diameter.
The old fence posts we had from hedge were so hard that you couldn't hardly drive fence nails in them, and lasted forever. I'm not sure I would want to drive into a row of hedge with a short shear on the front of a skid steer or small rubber tracked loader, I don't know how you would keep it out of the cab unless you had some serious guards.
At least a track loader or dozer you could have it out away from you, hedge is tough stuff, and with all the thorns, you don't want it in the cab with you. That stinks about the regrowth, but I don't know that the shear would be any better.
Whatever you end up with, you are going to have to have cab protection for going after hedge.
I had no idea what an Osage Orange tree was
You haven't lived until you've had a snowball fight with hedge apples, you can knock someone out with a head shot. My uncle's farm had a bunch of them along the creek. You sure don't want to stumble into one not paying attention.