The flights from SIN to PER on QF are depart 1930 (QF78) and 2330 (QF72)

As I normally would either overnight or connect from the CX flight from CMB (arrive 0600) I wasn't really too happy with the schedule change from 0900. So I suppose if you can't convince QF to let you book QF78 I would recommend an overnight. There are some quite nice 3* hotels than are clean and inexpensive. There are also quite a few 1* that are clean and outright cheap (with very small rooms)

The cabin lead FA should be able to assist if the seat issue remains unsolved by the time you board. Most people (at least in Y and non FT addicts ) aren't that picky about which aisle seat they have - as long as it is in the first Y cabin.

OTOH, I hope MH's entry into OW isn't delayed and that their fares are reasonable.

Happy wandering

Fred