Recently proposed theories for shear banding in wormlike micellar solutions (WLMs) rely on a shear-induced isotropic-nematic (I-N) phase separation as the mechanism for banding. Critical tests of such theories require spatially-resolved measurements of flow-kinematics and local mesoscale microstructure within the shear bands. We have recently developed such capabilities using a short gap Couette cell for flow-small angle neutron scattering (flow-SANS) measurements in the 1-2 plane of shear with collaborators at the NIST Center for Neutron Research. This work combines flow-SANS measurements with rheology, rheo-optics and velocimetry measurements to present the first complete spatially-resolved study of WLMs through the shear banding transition for a model shear banding WLM solution near the I-N phase boundary. The shear rheology is well-modeled by the Giesekus constitutive equation, with incorporated stress diffusion to predict shear banding. By fitting the stress diffusivity at the onset of banding, the model enables prediction of velocity profiles in the shear banded state which are in quantitative agreement with measured flow-kinematics. Quantitative analysis of the flow-SANS measurements shows a critical segmental alignment for banding and validates the Giesekus model predictions, linking segmental orientation to shear banding and providing the first rigorous evidence for the shear-induced I-N transition mechanism for shear banding.