JohnN:

Instead of a dog backpack, consider a folding luggage hand cart, one with large diameter wheels.

You can adjust the amount of load bearing weight on her back by positioning the weight of what she carries. The further back on the cart, less weight, the further forward, the more weight. You don't want the weight too far back as it would try to lift her.

If she is injured, it can be used as a dolly to carry her. If you have to lower her by rope, you can lay her on the cart and strap her down, thereby spreading her weight on the cart and putting the lowering connections on the cart instead of her.

For those times you go somewhere without her, you will have a luggage cart.

Good luck!

Bountyhunter