You may want to post about this in the frame-builders forum. I would think the folks there might have some experience working with 531, and be able to advise as to how weak or strong it might be after such a denting and straightening. It looks pretty good now, but I agree with your skepticism re future crumpling potential. If you have to repaint anyway, it may be worth replacing the whole top tube, and straightening the down tube and head tube alignment at the same time.

And it might also be interesting to try the freezing water trick with the blocks clamped in place. That might keep the expansion limited to just getting the tube back to round, without bulging or splitting. But I really have no first hand knowledge of such things. Do keep us posted. I do love this sort of project.