If I understood correctly, we install al v9 OpenPLi, then at boot, we have the possibility to install another image, like OpenATV, for example by downloading it directly from the OpenPLi image?
Correct.
I suggest you put an OpenPLi in flash (slot 0), as it is the smallest of all images, which is important as all installed images share the same file system. And then only use the other slots, so you always have the option to fall back to slot 0 if something fails.
With BackupSuite, all the images installed will be save ?
I have to pass that on, I never use it, and no idea how it behaves in a multiboot environment.