If you go to Mexico or Canada and do not get a new F-1 visa, then you always have the option to travel back to the US with your expired F-1 visa and your current valid I-20 and valid passport. However, you have to be back to the US within 30 days and cannot travel to a third country.

Getting new visas through US Consulates in Mexico and Canada is far easier than the US Embassy in KTM. For accurate and current information on this, it is better to consult your international student advisor and visit the USCIS website. I am not sure, but I believe, that the process to get the F1 is the same as you went through in Kathmandu no matter from where you apply.

Hope this helps. Good Luck to all of you who's considering this!