No I would not save the roots from this pot. Much better off with the new roots from the cuttings. Al

I agree that the pot is way too small & also that it's best to start again; these plants will put up with a lot but even they can't thrive in such conditions. I have no experience of trying to root whole stems: I usually break off the green shoots & root them, though it's true you only get small plants that way. Why not try both?
The reason it's impractical to grow these to really large sizes successfully is that you would need larger & larger pots (to support the weight as well) so you'd need lots of room.
But there is a very interesting post further up this thread about a real monster: "When to Water Extremely large Jade Cutting"

Oh, don't throw it away. Just put it in a bigger pot and give it a good soak! If you prune it nicely and remove all the crossing branches it will look a whole lot better. Good luck. Ian