If I've previously created a page in one form, and I want to re-use that design in another form, how can I copy and paste that part of the structure?
I've tried Ctrl-C in one browser tab, and Ctrl-V in another, but there seems to be no effect.
I'm reluctant to try and save out to a block because the forms are in two different organisations, and I don't really want to keep re-using this again - it's a one-off.
Copy and paste in Composer and Maestro does not use the system clipboard, effectively they flag what has been copied and when it is pasted it is the application that handles it including renaming duplicate names, etc.
You can shared the page/section as a component and then re-use in another form.
To share across organisations, you will need to export and import the shared component from one organisation to another and ensure that the component is published to an appropriate library in each organisation.
This technique has its own drawbacks. It's sometimes difficult to refactor content which includes sections. It's not impossible but requires a lot of work on the block to make it work in a new layout, requiring a lot of imagination on what might be required later. The fact is that sometimes the same functionality needs to work in different layouts, and currently, there is no support for this: I have logged some enhancement requests.