I'll give WS a rest.
1. Essentially no. Sure, you can apply, but Plastics is ridiculously competitive, and GS is very competitive, so your chances of success would be slim. Note that you absolutely must pass Steps 1/2CK/2CS before the rank lists are due, or you are automatically withdrawn from the match.
2. Only the programs you applied to the first time will know, if they can remember. If they filtered your application out at the first pass because you didn't take Step 2, I doubt they would even notice.
3. If you need a visa, then maybe. There is a thread about visa's stickied at the top of this page. An H visa requires Step 3. A J visa does not. In general, Step 3 won't make you more competitive unless you want an H visa (and a program is willing to give it to you).
4. No. There is talk about changing it, but it's at least a few years away. Perhaps combining Steps 1 and 2 into a "Gateway A", and then Step 3 becomes "Gateway B". But I don't see that happening soon, and certainly not by 2013. When they make the change, it will be for incoming medical students (and hence at least 3 years of notice for schools to adjust).