I am not very sure about it because I kinda out of touch with
the local unis admission already. But generally, institution will
allow you to defer your university studies when you're in NS and
when you're out of NS, you can start your studies at the
institution. I'm not too sure if this has change.
For NUS admission office, call 6516 1010
For NTU admission office, call 6790 5972
Yes, you can apply multiple institutions and finally accept one
of them.