The wrong step is precisely in the first line of your code. devel-su doesn't grant you all of the available privileges, even though you may be running open mode. If so, please install opensh from here and you are good to go, use it instead of devel-su. If you are in normal mode, you need Inception and aegisctl, then install opensh through Inception then loosen Aegis security a bit
and try to mount the chroot as you have before.