1) It really depends on the SAN. You have to find out if it's thin provisioned on the SAN. My older Hitachi's don't do this so it's all thick on the SAN (it would use all 600 GB if you carved a 600 GB LUN out).
2) I've never done this myself. I've used vmkfstools for other reasons. I'm pretty sure you need as much free space available as the size of the new vmdk file. To be honest the best way to "re-shrink" a vmdk in my opinion is to use storage vMotion or the VMWare converter and select a new LUN that has the space and then specify thin provisioned.
3) If Windows is reporting 100 GB free and the VMDK file is using 600 GB, and you move it with thin provisioning enabled it will use 500 GB on the new location.
If you have a cluster and other LUNs available the easiest thing to do is just storage vMotion and select thin provisioned.