Why will Jade loose all her powers with the Green Sun's destruction? Wouldn't she only loose the powers granted to her by the green sun and still have her base powers as a Witch of Space?

Oh, Jade’s only losing the Green Sun related powers, it’s just that those particular powers are very useful indeed. Moreover, Jade is used to relying on those powers, so I imagine that while she may learn to compensate quite well for no longer having them, it may take a long time.