If I would be you, I would go to Nepal and also get married after engagement in this summer. Since you are just a couple of semesters away from graduation, it doesn't really matter to me to get married before or after graduation. I don't think it's a wise thing to wait a long time to get married after engagement. It's from my personal experience. If you get married in this summer, you can let her apply for  F2 (student dependent visa) after preparing documents on your return back to US. If you prepare good documents and get lucky, she may come to USA by your graduation day. It's also one of her valid reasons for her trip to attend on your (husband) graduation ceremony and it also saves your second mandatory trip to Nepal just for the purpose of marriage. I brought my wife on F2 visa in 2006. I wouldn't take a risk to travel back to USA on OPT since we have heard a lot of stories at Port Of Entry. Just don't be that one. Good luck for your plan whatever it is.