Spoke to JPSC and they have clarified the following, however given how inconsistent they can be at times it is always better to verify.

1) Even though transferred miles have a 12 months validity if miles are transferred into a Plat account they will not expire as long as card holder retains Platinum

2) In the event that member drops to Gold or lower, miles that were due to expire during the Platinum period will come back up for expiry on the next quarter (i.e. if you tier expires in May and you don't retain - all miles that would have expired had you not been Platinum will expire on 30 Jun).

Hope that is useful to fellow 9W'ers - Finally another serious Plat benefit worth having.