FRS = RA amount $77.5k (from OA&SA) + property pledge $77.5k = $155k
If you celebrated your 55th birthday between July 1, 2014, and June 30, 2015 (inclusive), then $155,000 is your Full Retirement Sum, yes.
I'm interpreting this statement that you made a property pledge (or have a property charge) and funded your RA at what's now known as the Basic Retirement Sum ($77,500). But please correct me if I'm wrong.
input $5k (withdraw from SA) to reduce housing loan ($95k - $5k) which will flow to RA account = $77k (property pledge) - $5k = $72k (reduce property pledge amount), thus RA will increase by $5k (4% interest).
Why would withdrawing SA reduce your housing loan? If you use the $5,000 from your SA to pay down your mortgage then your outstanding mortgage is smaller. (This would not be a smart thing to do if your mortgage interest rate is below 4.0% and you have other, lower yielding dollars to pay your mortgage.)
If you use your $5,000 from SA to refund $5,000 used for housing from your OA then you're reducing your SA balance and increasing your OA balance. That isn't a good trade.
If you use your $5,000 from SA to refund $5,000 used for housing from your SA then you're losing one month of interest on that $5,000 and otherwise leaving your SA balance unaffected. This isn't a good idea either.
If you use $5,000 from SA to top up your RA (CPF can do this for you as a transfer if you wish), OK, you could. I'd try to find other, lower yielding sources of dollars to top up your RA, but it's allowed.
FRS = RA $77.5k + $5k + property pledge $72.5k ?
Yes, if you top up your RA (no matter what the source of funds) then your property pledge should go down. Does this matter? If you're not going to sell your house it doesn't matter at all. If you're going to sell your house and buy another one (in Singapore) it probably doesn't matter at all. If you're going to sell your house and not buy another one in Singapore then it might matter a little, but "matter" here simply means some of your home sale proceeds (or other dollars, from somewhere) need to go into RA, whereupon your CPF LIFE retirement income stream will go up substantially.
What problem(s) are you trying to solve with this idea?