Thank you so much all for your help and suggestions.
I have seen past papers and scoring trend of this subject and also considered my aptitude and now come to the decision that I should really opt Environmental Sciences that will also help me in other subjects too.

No doubt Environmental Sciences...gender studies is opinion based subject and will be low scoring plus you know the gender discrimination in Pakistan is always referred to women rights and you dont know who will check exam, whether he is feminist or not...

Meanwhile, if you are opting IR then opt international law instead of criminology and keep US history as these both subjects relate to IR and will complement it...

Yes, your argument seems right.
On the other hand, I have checked syllabus and past papers of I.Law.They seem quite confusing and a bit techincal to me.Should I take a chance with this subject knowing well these difficulties?

In the beginning all things which you dont know seem confusing to you but you learn to understand them by repeated practice and hardwork...once you didnt even knew how to read / write and now you are preparing for CSS exam, doesnot these things encourage you...

Now back to question, all international treaties and rules are part of international law while international relations of different countries form these laws with the demand of time and nature of conflicts...I didnt said this subject is ditto copy of IR or vice versa..but both have many similarities so you can easily relate one with another and it will help you while attempting any question as you would already have known the event, its historical background implications and what world decided to do to bring peace or whatever the interest groups wanted back then...

__________________
Believe you can and you're halfway there. "T. Roosevelt"