Kotobuki 2-Tiered Bento Box, Woodgrain/Cherry (Sakura) Blossom

What is the capacity of the box in milliliters?

is this microwavable?

Is there a way to get a replacement part for this? I bought one years ago that I would like to use and am missing the lid for the center tier.

Do you have to use the second tier for it to fit right? Or can you just leave out the waterproof tier?