From my experience, which is only with M. rubra - the young saplings have the deeply lobed leaves (nothing like the ones above) but once they mature they get less lobed. Even if my dh cuts the same tree down over and over, it will mature and the leaves change shape. Also common with mulberry, all of the trees branch out with woody stems, not up with many green stems, like the OP's. From what I've found searching, M. alba is similar. I think a good way to tell M. rubra from M alba is the leaves of M. rubra are rough like sandpaper, where M. alba is smooth. But that doesn't help anything in this situation.
Like I mentioned, I'm still hoping Rob, the OP will have new info about this plant.