I am assuming the mother is here already so I am not sure what is the reasoning behind giving birth in Nepal. Are u concerned about the finances or what?
If I were you, I would just deliver the baby so that he/she can be entitled for all of the benefits as a US citizen. I do not see any point of going back to Nepal and initiate the immigration process just to bring the baby back here. Even babies born from illegals are US citizens so I am not sure why you mentioned mom does not have a GC and stuffs.