That should work, as long as the copy of templates.js generated when you run "make templates" is the one dxr-serve is finding. However, if you ever accidentally ran "python setup.py install", you will have a second copy of DXR in your systemwide Python site-packages dir, immune to any changes you make:
https://dxr.readthedocs.org/en/latest/development.html#troubleshooting. I suspect this is what's going on from the tests you've already run, but, to corroborate, put a syntax error in app.py and see if you can get dxr-serve to crash. If not, you definitely have a second copy of DXR hanging out somewhere. You can remove it with [sudo] pip uninstall dxr.
Cheers,
Erik