Regarding battle backgrounds, a solid black background would work as a placeholder given the asset pack you used. It’s a small effort that would have gone a long way to keep the player focused on the battle itself, rather than be distracted by the black boxes.
As for the SV battler head idea, I just want to emphasize that that’s just an alternative when there’s some reason for not showing a face. Faces are generally better because they can be more expressive. In our game, the reason we don’t add faces to dialog boxes (during VN segments) is because we already have busts and didn’t want to divide the player’s attention. There could be other reasons for not adding faces, including if you just haven’t made them yet. And there are other alternatives you could do to help identify the speaker, like a tail from the dialog box to the character (may require a plugin). Just giving you some options because the more tools you have in your toolbox, the better you can adapt to whatever challenges or limitations you have in your next game.