I'm thinking about controlling the fan motor through a fan speed controller powered from a 20a 120vac 12ga. Circuit with the fan speed controller between the breaker panel and the fan outlet. I would like to control the light portion of the fan light combo through a 3 way/ 4 way/ 3 way in series between the source and the fan light combo which is powered from a 15a 120vac 14 ga. Circuit. My question is if it's reasonably easy to separate the "neutrals" (grounded circuit conductor) in the fan light combo to achieve this without having to change the existing wiring around. The 20a circuit would have a cable running to the fan motor and the 15a circuit would have its own cable running to the light portion. Thanks any suggestions would be appreciated.

Yes just run an additional neutral down the center of the fan.

As long as it's not a remote fan in which case you wouldn't be able to do this anyway.