1. Depends entirely on the application. What didn't work with v1? Adjust accordingly. I like to find something as close as possible to my needs and steal their design. I once needed a coin battery door that didn't need tools to access. I bought/found/had people bring in every remote control possible. Measured/copied and Bob's your uncle.
2. Clear ABS. Cheap, strong and better clarity of PE or PP. No reason to get fancy with a PC.
3. Accura ClearVue ( an SLA process) - http://www.3dsystems.com/materials/accurar-clearvue
Sand it smooth with 400 grit, hit it with acrylic clear paint and you will be surprised how transparent it is. Also has excellent flex without snapping. Very similar to ABS, but still more brittle.
4. I don't waste time with FEA. If it works in the accura clear, it will work in the ABS. Metal safe tooling will entirely depend on the design. But the snaps in the world that you pictured are not high tolerance. I like nibbling 0.002 at a time, but again, it depends.