Yes, you can conclude the process without coming to the U.S....sort of. Your spouse doesn't become a lawful permanent resident until your spouse enters the U.S. and is inspected. Then your spouse becomes a permanent resident of the U.S.
Yes, the process can be completed in Nigeria. But, the US citizen must come to the US at the same time as the alien immigrant.
PROFESSOR OF IMMIGRATION LAW for over 10 years -- This blog posting is offered for informational purposes only. It does not constitute an attorney-client relationship. Also, keep in mind that this is an INTERNET BLOG. You should not rely on anything you read here to make decisions which impact on your life. Meet with an attorney, via Skype, or in person, to obtain competent personal and professional guidance.