VERY impressive improvement from image one to the last one.
The lighting setup is your last problem.
Normally studio lighting it set up in a couple primary ways.
1. to simulate what we humans view as 'correct'. Which of course is the sun above us and the light reflecting off the earth.
2. to show the shape of 3d objects with hilights.
3. to show dimensionality with correctly placed shadows.
You should have one primary lightsource. Make it from 3/4 above, it can be moved 360 degrees untill you are happy with the direction. I would recommend for your setup from over one of your shoulders at about a 45 degree. The secondary lightsource should be about 2 stops below, making it a bit cooler will 'fake' the feeling of realistic lighting. (only a bit) This light is only to remove any black shadows and fill in the detail in the shadows.
The third light will be a larger area with an even lower output, this is to be positioned to reflect in the primary reflective areas. If you are going for a studio look, this will be a bright white light (like a softbox). The purpose is to reinforce the 'metalness' of the metal bits and to make them look yummy.
The fourth/fifth lights are catchlights (also called hairlights) traditionally they are low power spots (400w) or focused flash (low power), they are designed to hilight edges against darkened areas. Like an ear or top of the head. In your case pointing a few lights from behind to give the white plastic a rim light will describe the 3d shape even further. When photographing glass items, like beer, photographers often place a silver reflector right behind the bottle, it's unseen by way of it being completely obvious, it reflects light back through the liquid making the color of the liquid even more vivid.

In relation to your items, pointed a directed spot behind the objects against their bases will pump light INTO the objects and increasing the feeling of translucent white plastic. This will also add light (via internal bouncing) to some of the edges, in a 'internal glow' fashion that is quite interesting and appealing.
Pick up a book on studio lighting. It's the single best thing you can do to improve your 'reality' of your CG studio renders.
cheers.