The buckle inside does just hold the frame in place. You adjust the frame to fit your back by removing it and bending it to fit. It takes quite a lot of force to set it in place.
Remove it slowly though otherwise you get an electric shock :shock:
The straps on top of the shoulder straps do just pull the top of the pack closer to your back, making the load more stable. On larger packs similar straps are found on the hip belt too, to do the same at the bottom of the pack.
Carry your axe as you would an ice axe in the loops provided.
Try not to end up with kit strapped to the outside of your pack, if you are having to do this then you need a bigger pack.
I have a karrimor sabre 60-100 that I use in winter, when carrying thermarest, bivi bag and warm clothing. And in the warmer months use the Sabre 45 and replace thermarest and bivi bag with a Thai hammock, and the winter clothing with more appropriate warm weather gear.
I used to own a Rocket Pack, but the the sabre 45 is a much tougher better made pack, and is considerably more comfortable, especially with heavier loads and over long distances.
The rocket pack takes the weight on the shoulders pulling you backwards and quickly tiring the shoulders. Whereas the sabre 45 supports the load on the hips (the bodys natural centre of gravity) making it far more comfortable.
Hope that is of some help