If I were you, I would not take a chance. The EAD card that comes with your OPT approval clearly states "it doesn't warranty your reentry to the states". F1 extension stamping for OPT from Nepal is not a very safe route. If I had to put myself in your shoes, I would ask for my employer to apply for H1B after I start the work in April. Once I get H1B approval notice, I would go Nepal in December and come back with H1B stamped.