I definitely hear "Born as my leader" as the last line, which would fit with the whole "Byleth is a demon's name" theory if Byleth does end up with a leadership role.
When I first saw that it I thought it looked like a spine, but you're definitely right.
So the two are definitely related, but there are a lot of possibilities as to why. Maybe the bone look is intentional?
A small thing to note, only 5 of the 7 options have a watermark.
Maybe those are the actions that end the unit's turn? If so, it would imply item use doesn't spend an entire turn anymore.