Why do you believe it's risky?
No. SA will be a little above $40,000 since you must leave at least $40,000 behind, and due to the fact you can only buy T-Bills in $1,000 face value increments you won't get down to exactly $40,000.
Sorry, not allowed. Once your SA reaches the Full Retirement Sum, inclusive of CPF Investment Scheme (SA), you cannot make OA to SA transfers. (Wouldn't that be nice, though?)
No, I'm afraid not. It'll also be horrifically expensive because of the loss of Special Account interest on the T-Bill (~8 months of SA interest lost on a 6 month T-Bill purchased at original issue and held to maturity). And you still presumably have compulsory contributions streaming in. If you raise the shield months or even years before your 55th birthday, you won't be shielding the compulsory contributions that land in SA thereafter.
The low volatility Singapore dollar bond unit trust purchased and then disposed of quickly via a zero fee platform is really the best way to shield. You can run a "dress rehearsal" a month or two before your 55th birthday at whatever the unit trust minimum is (S$1,000 typically, evidently).
Yes.