I 485 doesn't have to be approved in order to stay in US. Just search few past threads in sajha, there are tons of nepalese lawyer they can help you. If you had filed I 485 along with I 130 I am 100% sure that your mother can stay over her B2 visa period until I 485 gets approved but your case is bit different as you are filing I 485 after applying I 130. Again try to find good lawyer or do some google search I am pretty much sure there are many other who has been through this situation. Good luck