Entry in SVP before DP approval

She can travel with you using visiting pass. As you didn’t mention about your salary, so I assume your fixed salary per month must be S$6,000 per month in order for you eligible to apply DP for your spouse and children.
If she comes as a tourist using visiting pass then she has to show her return ticket. If you don’t use that ticket then later can change to another date.

DP won’t take much time other than her certificates verification.

She is entering using SVP has no direct impact to her IPA letter on student visa. She is traveling with you as your spouse and later she can get her student pass and IPA. If during her stay (you can extend further 30 days or 60 days for her), she couldn’t get her Student pass then she has to leave the country then return later. Good luck

Thanks Surya for the reply. Yes, I am eligible to apply for DP.

A bus or ferry ticket to Malaysia or Indonesia is a cheaper option than a flight that she might nut use in the end.
But if she shows the DP or student pass application and explains that she hopes this would be decided positively during her stay - and that, if not, she would of course book a return ticket and leave the country in time before SVP expiry - she might get in without needing a return ticket.