I'm not sure if we want to prevent create in this case. In fact we more or less consistently go by "just do it and adapt", or "do the hard work and make it simple".

I'd argue that pasting in a scenario like the following should just work, just with message flows not pasted:

But it does not 😢

I'd argue that pasting in a scenario like the following should just work, just with message flows not pasted:

I was considering that, too, but wasn't sure if that would look broken.

What should happen if some elements are already present on the diagram? How about if the diagram is already a collaboration vs. a process diagram with elements? If we are to create missing participants, should the referred processes be executable or not? Which participant should refer to already existing process?

I am not sure about the answers to the questions above. Therefore, I'd suggest to start small with message flows removal. The user can always wrap elements in participant(s), but moving from collaboration to a single process diagram is much more cumbersome.

I discussed this again with @nikku. We concluded that removing the message flows is the least intrusive option for now. Copying, as shown in the recording, is considered an edge case. We would assume that you either don't copy message flows or you copy the participants as well. We're going to remove the message flows on paste if the participants weren't copied.