This is one approach to allow non-js validation when submitting edit conflicts. The idea her is to resubmit the form to the editing workflow of the EditPage.php. When we see, that there is a missing selection for one of the sides, this will be done using the original base revision as edit revision id.
In that case the algorithm should then trigger the edit conflict "again", although it might already consider partly solved aspects of the conflict as well as third party edits happening in the meantime. See an implementation that tried to realize this idea .
The ticket is mainly for documentation. At the moment we did not decided on implementing a specific approach to solve that problem