Absent IRROPS, UA almost certainly won't proactively SDC an SQ segment on a paid 016 ticket. SDC does not cover OAL metal.

For an Award ticket if X/I class is available, it may be possible to reissue with the desired SIN-KUL flight given UA awards ex-USA have no change or redeposit fees, though this may depend on preserving UA1/29 award availability

On whether SQ can touch a 016 ticket, I've taken earlier flights before (e.g. BKK-SIN) on OAL ticket stock (including on 016 award tickets). It's kind of like the standby concept. But it's not guaranteed. In any case an additional hour or two at Changi is not the end of the world