Very cool tops indeed!
Your bottom and sides could easily be as thin as 12 mm, but making them thinner would rule out the option of rounding off the body sides.
I don't think that. If you leave extra wood at just the corners, you can pull off a round over. I would have the top and back be thicker than 12mm then route all but the sides to the desired thickness, and that will leave enough for rounded edges. On my semi hollow build, my top and back are 1/4" and the sides are a bit under 1/2". I also left wood for arm and stomach contours, and a neck transition, but the rest is pretty much hollow. The lower wing feels to be about 1 lbs roughly, but i would need to borrow my friends fish scale to weigh it accurately though. I used a maple top as well, but with flame figure, instead of spalting. the mahogany wood i used for the core was pretty light to start with though. What is your core wood?
Now i need some feedback from the LC community.
ohh well i know better for next time. 
which makes it hard to see the blackline unless you hold it at a certain angle.