Now we have a nice side-by-side diff in the 'suggested edits' review pages, could something similar be made available as an option when editing a post?

It wouldn't be interactive - a button to get the side-by-side diff of the current state of your edit versus the original. This would be useful when shuffling lots of whitespace around - a final check that you haven't broken the code.

Work around at the moment is to submit the edit, then check the edit history. But that isn't a side-by-side view.