You can export to .shapr format and import it to the other workspace.
No, currently, the only way to duplicate a workspace is to export it.
Infinite. But you are probably asking this because most of the time it is really hard to modify bodies once you created them. But this release: http://discourse.shapr3d.com/t/something-big-is-coming-and-we-cant-wait-to-share-it-with-you/ is going to fix that. Also, undo history is not saved, so if you quit the app it will be lost.
Not sure I understand, could you please elaborate on this?