I do not understand why gravitational lensing should us enable to differentiate between dark matter and deviations from general relativity on a long range scale. Both the movement of stars in the outer rim of a galaxy as well as gravitational lensing are caused by the same curvature in space that we call gravity. If a theory explains this curvature without dark matter it is still the same curvature given by dark matter and such gives the same effect of gravitational lensing. So I would expect movement of stars (and galaxies) and gravitational lensing being consistent with dark matter attributed to those galaxies and with the modified general relativity. Both would (and need to) give the same curvature of space resulting in the same effects we can see.