It’s tough to piece it together. First glance variation with frequency suggests resonance excited by something that varies with running speed, but what? (the $64,000 question).
You mentioned OEM suspected endshield resonance. I have seen that on large motors. Typically the end of the motor has a big plate or inspection cover on the inboard and outboard end, something like a semi-circle above the bearing. Typically it vibrates with a drum-head type resonance and it can easily be verified by bracing (or sometimes just touching) the center of that piece… which (in our case) brings the vibration down all around the machine. Your geometry may be a little different, in that case try mapping the axial vibration around on the end surface of the motor and try temporarily axially bracing the place with the highest vibration. If you confirm that type of resonance, the fix may be as simple as providing some stiffening to that piece.