[Income tax] Topping up parent’s CPF SA and asking them to withdraw immediately.?

You won't be able to do so as Retirement Sum Topping Up after 55 will have the monies flow to your parents RA instead, which then converts to a higher CPF LIFE payout when they start receiving it. Also, as you mentioned, if your parents CPF RA have more than the FRS amount already, you also won't get tax relief if you top up to their RA.

So you'll want to make a top up to their CPF account via 3 account contribution. The trick is, if your parents MA is at the BHS (currently $60K), then contributions that would have been supposed to be allocated to the MA will then spill over to SA/OA, so effectively you can treat CPF OA/SA as a high interest savings account. But tax benefits will not be possible this way.

If only it was that easy. Firstly, you can't top up to SA directly after 55. Only to RA. And any top ups to RA would convert that money to additional CPF Life payout. So can't take out immediately.

You'll instead need to do a Voluntary Contribution to all 3 accounts with then a small portion paid to OA and SA. But that isn't tax deductible. But the OA and SA monies can be withdrawn if FRS was met.