I think there is a problem with moving a complete node tree, as not everything in that tree is worth keeping - if I post something as a reply to a Scrap Node, I maybe don't want it to be kept or even voted on :). Then there also is the "problem" that one gets no XP if a Scrap node tree is made permanent. On the other side, such is life.
As for the pasting issue, you always have access to the source of nodes created by you, so you can cutīnīpaste the source from there into the new node... This won't help if the creating user went offline before the decision for node permanentness and copying all temp nodes was made ...

I see need for more discussion on that topic !

Well, without changes to the engine, we can't prevent people from linking into Scrap Nodes and maybe it's even helpful, for example for posting a link into the chatterbox.

Your solution would be the most elegant solution I guess, by allowing replies to Scrap Nodes, we avoid the multiple-access issue completely.