I did have a blue Platinum Cool with a medium nib a while back, but recently sold it. I'll try to answer your questions to the best of my knowledge.
1. The definition of a demonstrator is a pen that shows its internals. It would make sense that all demonstrators (not just from Platinum) are transparent/translucent. The demonstrators are the ones in clear, blue, and magenta.
2. There are no internal differences between the Cool and Balance, only the trimmings and body colours.
3. They are very durable. I would say between a Pilot 78G and Lamy Safari. I've dropped mine many times and have been carrying it in an ordinary pencil case for months.
4. No, I sold it already, sorry!
The nib is quite springy, but no where near as much as the Noodler's flex pens, Namiki/Pilot Falcons, or the soft nibs from Pilot. It's pleasant to write with, but don't expect too huge of line variation. If that's what you're going for, make sure to get the fine nib version, as the line width differences are more noticeable.
I hope that this helps!
“My two fingers on a typewriter have never connected with my brain. My hand on a pen does. A fountain pen, of course. Ball-point pens are only good for filling out forms on a plane.”