Hi Smelly,
Good to know you are keen to pursue this line. Lots of things to think about so spend some good time going through it all.
The STAA program is a self-sponsored program. Means that you get a CAAS CPL/IR at the end. STAA ties up with Tiger so what Tiger may do if there's vacancy is that sometime during your ground school phase they will interview the students that are interested. If you are successful then you switch over to the MPL program and get a job with Tiger on the condition that you successfully complete the course. If you are not selected then you continue with your CPL/IR as planned.
As you have just completed your NS I assume you don't have a university degree. Airline cadet programs are very competitive and in the absence of your flying record they will then browse through applicants' academic records before doing the selection for interview. So if you have a university degree you then stand a better chance of being called up. Also a university degree acts as a backup, in case for some whatever reason you don't get to fly.
Massey and UNSW programs are good, albeit expensive. Yes you do get a NZCAA / CASA licence. That depends on what you want to do with it later. If you insist of coming back to Singapore, then you will need to convert your licence to CAAS licence which involves rewriting all ATPL exams, flight test, and an employment contract with a Singaporean air operator. Remember all these are subjected to changes. If you don't need to come back to Singapore then you are free to use your licence to find other flying jobs around the world. Just need to convert to the local licence if you're not flying in NZ / Australia.
I spoke to a Massey representative recently and she mentioned that the coursework you do for the degree is their ATPL equivalent, so you do not need to spend additional time doing ATPL ground school.
All the best.