Yes, it does relate over the storage-backend interface.
if the custom charm was deployed after (not part of the bundle), then that would be explanation why the LVM backend was enabled, and then disabled when the relation was added.
you can add your custom charm to a bundle with something like this:
applications:
...
cinder-hitachie590:
charm: ./cinder-hitachie590.charm
relations:
- [ cinder-hitachie590, cinder]
...
How can I deploy my bundle without any backend or LVM so I can later on deploy my custom charm?
based on the source code I pointed out, this is not possible, the comment left in that section this is because otherwise the service crashes and gets restarted over and over again. This could be considered as a bug, feel free to file one at https://bugs.launchpad.net/charm-cinder/+filebug