In the Items view, we can Cut and Paste but there is no Copy command. Cut+Paste will cause a Move operation.

Optimally, I'd like to be able to "fork" an item (at least a file) by creating a new Copy/Clone of the item that shares a common history with the original item from the cloning point and backward in time, but that has its own unique history from the cloning point and forward in time. (This was possible in SVN, where it was possible to trace the history of a copied file back into the original file where it was copied from.)

AFAIK, such a Copy operation seems to be missing from Plastic which can only do a move operation. I can see that there may be technical reasons for this, but it would be very nice to have that feature in Plastic. And possibly, implementing such a feature may provide a key to solving the problems described in the following popular uservoice:
http://plasticscm.uservoice.com/forums/15467-general/suggestions/3311147-possibility-to-merge-independently-added-files-dir

That said, even if you are not going to implement a true Clone/Copy operation you could still add a Copy command in the Items view. It would then simply Add a (history-less) copy of the item when doing Paste (just as in Windows File Explorer). It would make these operations more symmetric and consistent. After all, it's a bit strange that we can Move an item inside Plastic but we need to go to the Windows File Explorer to make a simple copy of the same item...