I think that the with and without build-backend cases do not really need to be distinguished here. I think we should fall back to PEP 518 and invoke setup.py develop in case a build-backend.requires key is found, and otherwise just invoke setup.py develop.
This is not like the case with setuptools.build_meta:__legacy__, where we have some behavior that we want to deprecate, this is just a feature we haven’t gotten around to standardizing yet, so we can’t really implement it properly. We should be doing our best to make sure that editable installs continue to work as they did before to the best extent possible. Under the hood the implementation will probably change once PEP 517 gets the capacity to handle editable installs, but I don’t think that should affect users for whom it was already working.
For people who rely on PEP 517 for the build editable installs were already properly broken, so they are seeing no change anyway.