Yep, you should try that.
Sorry I thought backupmenu had a shell on its own (like backupmenu-multiboot has when pressing 's').. For sure kernel with frame buffer support is needed and a little hack in /sbin/preinit to enable fbcon.

About cssu: please have a look in the appropriate threads. But afaik going from stable to testing is no problem, just change the repositories (install cssu-testing). Going back is not possible.
I propose you directly go to cssu-thumb