Hi! I don't usually post anything on this forum, but this is something I might actually be able to help with.
Legs should not be out in front in a Stock saddle, definitely go with the more vertical lower leg. Every campdrafting comp I have seen have a general english seat, although they do tend to slouch a little bit but I think that's more us being lazy haha.
I learnt to ride in a proper Aussie (we just call them stock saddles here) and rode in one for 4 years. It was the longer flap, deep seat with mickey mouse ears type. No fenders or horn. In fact I haven't actually ever seen any of those here I don't know if people use them too often?
I was always taught the shoulder-hip-heel line and that if I look down I should see just the tip of my toe. In fact when I used a dressage saddle with the same instructor I was never told to change how I sat.
I now ride at a place that has only english all purpose or dressage saddles and I've yet to have anyone come up to me and say they can tell that I learnt in a stock saddle.
The type of saddle I learnt in
I'm on the chestnut arab, my best friend who rides western now is on the grey arab (this was when were 13??ish)
Me in the dressage (I would have been 14)
Me in an all purpose recently, just for kicks! I know I'm not perfect, just wanted to show how similar the riding is :)
A campdrafting comp I watched recently, gorgeous horse. You can see the slouchiness here but legs are underneath.