1. The government doesn't really give a shit as long as it is accredited and recognized by the assessment body for the occupation your son will nominate (Read the SOL list to find out)

2. No, see answer number 1.

3. I'd say medicine to be honest but even with a medical degree and 2 years worth of housemanship you still need to score 8s on your IELTS in all bands. You're better off being an electrician/instrumentation techie or plumber.

4. Don't bother, immi.gov.au has all the answers and they won't sugar coat it.
I think they've recently mooted removing accountants, and that engineers might be next on the list.

Remember that trade qualifications are assessed by the TRA which will take into context work experience, certificates, skills and licenses. They aren't as rigid as a professional body.

Singaporean experience is not viewed any differently than Malaysian exp to be honest with you.