Since it only 2 weeks, I think it is safe to book a single round trip ticket between the U.S. and Pakistan, on Etihad, Qatar, or Emirates.
I like Etihad for this because of CBP pre-clearance in Abu Dhabi. You will get processed by CBP in Abu Dhabi, and if there is any confusion about your extension letter, Etihad in Pakistan might have experience getting a hold of CBP in Abu Dhabi to answer questions.