Re: ...Nikah vs. Engagement...
the nikah is the real deal - you are married - you are 'halal' for eachother. The rukhsati is just a cultural thing. The husband does have to give a certain amount of "mahar" before consummating the marriage. He has the option of giving the mahar in full or giving a part of it and deferring the rest. From my understanding part of the mahar needs to paid before consummating the marriage.
I have also heard of stories like the one Faisal bhai told of a woman getting pregnant while nikah-fied. Only in Pak would it be considered a "sin" or an "embarrasement" - another example of how culture takes over religion.