If your wife is in the U.S., then you should consider returning to the U.S. to process her through the adjustment of status process before she leaves. Otherwise, she may have to stay out of the U. S. for upwards of a year. There may also be other complications with processing, where she omitted information when she applied for her visitor visa (e.g. that she was married to a U.S. Citizen).
As far as the other issues, you must intend to return to the U.S. and prove it to the satisfaction of a consular official. You must also earn enough to support your wife in the U.S. based upon 125% of the poverty guidelines established by the U.S. If you have failed to file Tax returns, then this can complicate matters. You may also need a joint sponsor, which can also create a barrier to getting your wife her lawful permanent resident status.
The U. S. Embassy in Lagos can be quite unforgiving of oversights. It is unclear when or how your wife got her visitor visa. I strongly recommend an appoitnment or teleconference with a competent and experienced immigration attorney before an unintentional mistake is made. Good luck.
This is general information, not legal advice, and does not create an attorney client relationship.
You can file the petition for your wife even if you are away in Africa. I am not sure whether you want your wife to do an adjustment of status, or whether you want to do consular processing. The process is very different for each. You dont need to maintain an address in the United States, but you need to prove that you intend to move to the United States once your wife recieves her residence. You also need to show income to demostrate your wife wont be a public charge in the US. Moreover, you need to show it's a real marriage, therefore you should gather documents of things you have together with your wife, such as bank accounts, loans, leases, etc. You should contact an experienced immigration lawyer to further evaluate your case.