(1) I would rather drag the mortgage payment if its HDB, since you can't reverse mortgage or refinance it (once its below $300,000) easily.
(2) if your plan is for mortgage, it makes sense to use OA monies to pay off HDB (since it only earns 2.5%, but loan is 2.6%) after you hit FRS, because if you hit FRS, MA contribution will overflow to OA once both FRS and BHS reached limit (so you are using interest from MA to pay for mortgage too), and you are unable to transfer to SA after FRS is reached.
(3) accrued interest is a double edge sword, but i would keep that option open (since if i have a windfall suddenly, i can payback anytime), and do RSTU (if there is tax advantage) / VC3A (higher priority if no tax advantage) or even invest cash. But if your willingness to take risk is low, go for VC3A, then transfer OA to SA first, until it cannot be transferred, then use OA monies to transfer to parents RA (1st priority, since can reduce allowance given to parents) / spouse SA (2nd priority since money is also stuck until 55) while keeping some for mortgage payment.
(4) investing with cash imho should be done young first, since you got higher ability to take risk (longer horizon to retirement, more liquid, and compounding for investment work better than putting into SA).
But of course, if you have $500,000 capital already... then what i say is not useful because you have too much money on hand (but if you transfer OA to SA, means not much).