In the investigation of most round-tripping bugs we need to figure out if VE modified the DOM in unexpected ways or Parsoid had issues in handling an unmodified DOM.
We need an easy way to display both the original HTML string retrieved by VE and a serialization of the modified HTML the VE sends out, possibly popped up in two textareas on some keybinding for easy copying.
This would at least allow basic manual debugging.
Optionally, as a time saver, it would be nice if you could also output a DOM diff that makes it easy to spot DOM differences. You could probably base this on the Parsoid DOMDiff module. This DOM diff can also be used in sanity checks to automatically detect VE round-trip issues.