Hey there... about two years ago, I was in the exact same shoes
as you are now. Well, at least the graduation, haven't-done-NS-yet,
haven't-taken-NAPFA and graduated-from-SP bit. 
With regards to your application, it varies, depending on how fast
they process your application. If it is processed slightly late,
you could be joining SPF after your 3-month BMT.
If it is processed even later, then the interview and whatnots will
be arranged sometime during your NS life. DOn't worry. If it falls
under the latter, just sepak to your OC or equivalent supervisor,
and they'll let you have time off to sort out the SPF details. No
worries mate. All you need now is time. Although it could be long,
but it will be resolved.
BUT... there is also a possibility of them processing your
appilcation quickly, and as a result you will receive a letter from
MINDEF if that happens. The letter will state that you will be
"exempted from National Service on one condition: that you remain
as a regular in the Singapore Police Force". If you get this
letter, then it really means that you've secured a place in the
SPF.
You've already cleared your 2 interviews. Have you gone for the SPF
medical checkup yet? Once they've sent you there, and you're issued
with a clean bill of health, then you're already in. It takes about
2-3 weeks for the results of the checkup to get back to you
though.
But if you ask me, I think you'll be coming in soon, without having
to go to the army. My reason for thinking that is that you've
already cleared your interviews. 
Regular intakes don't have prep courses. You will all be enlisting
on the same day, regardless of your NAPFA results. Depending on
your intake, there could be a few women in your squad, or you could
be like me: a squad full of young boys and every instructor mistook
us for NS boys. 
Your
squad will have people who ave completed their NS too. You might
even get CNB trainees.
And there will also be some people who will decide at the aslt
miniute that they don't want to join. So on the bond signing day,
you will see people who will not appear even after their names are
called upteen times.
And there might also be some who resign midway through the
training. 
But I digress lah. Anyway, to answer your questions:
POBC standard is 6 months: 3 months basic stuff, and another 3
months of NPCO training. But depending on logistical and admin
matters, it could stretch to 7, 8 months (beware: it almost always
does stretch). There are also those who sign on directly for Police
k-9 unit and Police Coast Guard. The PCG people will do 3 months
before moving out, while the k-9ers will be with you until you
complete training.
Training is subjective I suppose, but I believe you will enjoy it.
It will prove to be interesting. And what the recruitment guy said
is true: if you've cleared your interviews, then the checkup is
almost like a mere formality.
But your uncle is wrong about going to CID though. Usually NPCOs
get stuck for more than 5,6 years before they can leave. And CID
people usually come from Land Division Investigation Branches. They
usually have to serve for many years in Land Division Investigation
Branches before CID even considers them. This one, though, applies
to Junior Officers lah.
for senior officers, it's different. They have to move around
within the force quite regularly, especially the scholars.
Anything you'd like to know, feel free to ask me man. I'd be more
than glad to help with your queries.
