Relationship between flapping and lagging frequency
I agree with your assessment. Have you discussed this with your prof/instructor? When performing Rotor Track and Balance, a worn lead/lag damper or dampers will directly affect the 1 per revolution lateral vibration with little to no affect to other rotor rpm orders or suborders. This is most pronounced in helicopter models which utilize hydraulic dampers as opposed to elastomeric or friction type lead/lag dampers. Hope this helps.

Another factor that contributes to Lead/Lag is alternating spanwise distribution of lift as a result of forward flight. As a rotor blade advances, the lift migrates toward the tip. Conversely, as the blade retreats, lift shifts more toward the root of the blade. Still a 1 per revolution effect.

Chordwise blade mass distribution (Chordwise Balance Aka Product Balance) accounts for angle of attack changes with respect to power changes. If one blade climbes or dives a greater amount than the other/s in transition from Ground-Flat-Pitch to Hover, there is a chordwise balance disparity, which, in turn leads to effects in Lead/Lag. Still a 1 per revolution effect.