Since you seem to like sanding the gs, let me suggest getting some milliputty (if that is readily available where ever you live), and mixing that with GS. The sculpting properties are almost like GS, but when cured it is much more rigid and easier to sand.
Also, a good way of making cloaks is getting two sheets of parchment paper (one used for baking), apply some lubricant to them. Then flatten a piece of greenstuff (mix milliputty if you want, but its not neccessary) between the two sheets. Let the GS cure 30min to 1h. Then cut it into a shape of a cloak. Since the GS is partially cured, you should get nice clean edges without streching the stuff. Remove the parchment paper at this point.
Now you have a totally flat piece of GS in a shape of a cloak. The GS should be cured enough so that you wont leave fingerprints on it, but you can still bend it to a wavy form of a cloak. Getting the timing right might require some practice. It is easiest to superclue the cloak on the figure, since the GS wont stick at this point anymore.
The cloak might still be a little too flat for your liking, but you can always sculpt folds on it and such. Here is a example. Sorry for a bad pic.