Well, DiGi Prepaid I Like: Super SMS has free SMS whereby you only spend RM 0.50/day on SMS and you will get to SMS all DiGi numbers for free for that day.
MTV Powerpack is only for Friends and Family right?
DiGi prepaid I Like is for all DiGi numbers.

Well, why not for supplementary to mainline? They will be happy for that.
As for postpaid to prepaid, there is a way.
Just that it is quite hassle.
First, you switch via MNP to other telcos first, then MNP back as prepaid.
If you wanna do that, you must be mainline on DiGi first.
Supplementary line -> Mainline -> Telco X prepaid -> DiGi prepaid

Yes, you can always change back to DiGi, this applies to other telcos too.
There are forummers who tried this and it works.
There is no minimum of period that you must be with a particular telco upon your MNP.
You can jump to any telco, if you wanna try out everything.

How old are you now? Assuming your mum still pays for your bill, you must be quite young.
If I am not mistaken before reaching 18 years old, you cannot register a mainline for yourself.

Then you can register for yourself then. Consider nicely first.
Anyway, I only let my parents paid my bill until when I was 17. After that, I settle myself at university.

I bought a HP with that voucher 2 years ago. But if you use the voucher, the price will be discounted from the Recommended Retail Price which could be much higher than usual. So, check carefully the price to see if it's really worth it.

Digi Specialized store wont sell AP set. They mostly sell Ori set and the price is normally higher than normal market price. So, u got to check whether it is worth the price.