Dear Kamran,
With such a small moment arm, I would guess the heave displacement would only be minimally effected by pitch and roll motions when comparing the motion relative to the MSL versus relative the CM.
Alternatively, you could keep the platform reference point at PtfmRefzt = 0 and shift the outputs to the CM with a bit of post-processing, as discussed, e.g., in the following forum topic: Time domain analysis by AQWA - #2 by Jason.Jonkman.
Best regards,