Part of the goal was to have a large image of the remote person’s face at a height that’s comfortable for interaction and conversation, which necessitates a screen mounted fairly high up.

To your point, I bet a picoprojector mounted down low could be made to project onto a lightweight screen in place of the tablet.

(The tablet arrangement worked well enough in practice, and we used a tablet that had already suffered child-related damage)

Ah, you're right, I forgot about being able to see grandma's face.