Is it planned that the new editor should be capable of rendering what the current editor renders, and in the same way? That's a design decision that will have been taken early on. However the WMF interprets "Agile" (and I think the prevailing interpretation is not an entirely productive one), the answer to this cannot reasonably be, let's write some code and see whether it does or not. That decision, and those differences, will have to have been made clear early on, so that the community can decide whether it is a blocker (see discussions around Technical Collaboration Guideline/Community decisions) and if it is agreed by the community that they are willing in principle to accept differences in behaviour, and agreed the scope and scale of the differences proposed, then there needs to have been a discussion leading to a clear description of what will and will not be rendered the same way by the two software systems. It would now be useful to see the records of those discussions and those decisions.
In the case of the specific debate I mentioned, there is a non-meeting of minds because these issues have not yet been surfaced, and the the resolution has to be that if the new editor is supposed to render HTML in the same way as the current one, then the behaviour reported is simply wrong. If the new editor is supposed to render some input the same and some differently, then the behaviour here may or may not be wrong, depending on the proposals for the scope of the differences to be tolerated between the two rendering engines.
So: is the new editor supposed to render the same code in the same way as the old one? If not, what is the scale and the scope of the difference? Simple questions. I hope someone can publish the clear, definitive and agreed answers.