1. Yes you can because you're not cutting or breaking anything, just need the time and effort to play around with the mechanism.
I found a youtube video of it
www.youtube.com/watch?v=3IjSPL584zg
but I don't advise you to use metal tweezer though, it can easily scratch the mirror if it slips off inside the body.
One of the links in the Flickr page that I posted earlier also showed a step-by-step instruction.
2. Yup, so instead of the blank clear view of the viewfinder you'll always see the split at the centre, but it's not distracting at all to me and it's so useful I won't change it back to the original screen.
3. I got it second hand from someone and I didn't ask where did he buy it from, but some people have said that Orient Photo in Sim Lim sells it, and they'll change it for you too.
The shim is basically a small rectangular brass frame between the screen and your camera, it's just a spacer of certain thickness so that the autofocus of the camera is exactly the same as what you see in the viewfinder.
When I first installed mine I used back the original shim and it was too thin. The view would be perfectly focused in my viewfinder but was slightly off in the resulting picture. Same thing when I was using my kit lens, when the autofocus locked the split image on the viewfinder was a bit off but the picture came out sharp.
I modified the shim by adding a small tape to increase the thickness, just a small change in thickness. Some other people have tried using paper or clear plastic and some screens do come with replacement shims. It's really try and error at this step whether you got the correct thickness and others can live with the slight difference in focus because it won't affect the image that much but luckily for me it was perfect after that. Viewfinder, autofocus and final picture all show the same sharp focus.