Good news. I have worked on a page that was needing layout adjustments for each device.
I have successfully exported the JSON file and imported it to the live site via the backend editor. The outcome is success!
There are the following points that might be worth mentioning...
I had a struggle with the frontend editor interface, sorting out and revealing required options to adjust layouts. I wish there was more consistency in the location of settings such as padding. Sometimes it is under the Style tab, other times not. I would also prefer the default padding option to be 'unlocked' instead of 'locked'. Then again, I suppose other users might prefer it to be locked You can't please everyone all the time.
For a while I struggled to locate some of the element option buttons. It turned out they were hidden in the narrow portrait mobile and tablet views. It wasn't immediately clear that the buttons can slide out of view. Now that is understood it is easy to slide within the element to gain access to the options. It would help if the frontend editor offered some clues such as a slider. Just a thought.
The most useful option is the Simple width percentage adjustment. That helped a lot!
There was one final anomaly that had me going round in circles for no reason. One of the frontend editor portrait views (if I remember correctly it was the portrait tablet view), was showing far too much space above an image (i.e. within the image element container.) It was in the second of two columns, arranged vertically in the tablet views. Nothing I did would close the gap. There were no padding or height adjustments entered. However, the gap was not noticeable during testing once the JSON file was loaded to the live site. So it appears to be an editor view issue.
That said, yes we can end this topic now. If I strike any other problems, I will create a new post.
Thanks again.