Jacmel87

When a child is adopted before their 16th birthday, and receives an immigration benefit as a result of that adoption (such as being petitioned and obtaining a green card), they are considered the “child” of the adoptive parents. This means, the adopted child cannot petition for his or her biological parents or siblings, and they cannot receive any immigration benefit from them.
 
 If you obtained your green card because of an adoption, you cannot petition your biological parent, or your biological brothers or sisters.

Adopting a child who came on a visitors visa circumvents the requirement of living with the child in their home country for two years.  Child's biological parents whereabouts are a key factor in immigration adoption.  You will need a lawyer as you have already tried to get around the specific guidelines.

adoption rules for immigration benfits such as becoming a resident or citizenship are much stricter than just adopting. it is possible to adopt a person and have them not eligible to immigrate if they do not meet all of the qualifications.
 
what was your relationship to the child before you adopted them? 

You are going to base your decision about whether or not to marry her on what strangers on a forum say? That sounds odd. If you are ready for marriage, get married. If not, then don't.
 
In terms of immigration you have two options - a fiancé visa (whereby you MUST get married within 90 days of her arrival here with said visa) or spousal visa (if you get married first then file the petition for her visa). Read through the guides here for the pros and cons of each. Neither is perfect. 
 
You will also need to research the requirements for bringing the child here - in most cases the permission of the other birth parent is required. Or is she planning to leave the child behind?
 
I do have concerns with the amount of money you are sending to someone you have known less than a year and seen only twice. She survived before you were on the scene so she doesn't need your money now that you are in the picture. 
 
Also, you are still in the blissful honeymoon phase. We've all been there and it's great. But the reality of living here as an immigrant, especially for someone from a completely different background, can be very stressful and difficult. Now more than ever. I've had struggles and I'm British and had visited over 40 times before I moved here. Be prepared for some very tough times. Have a read through the more ugly threads here to see what I mean. It's not all rainbows and unicorns. 

What anti-immigrant environment????  Maybe you meant to say anti-ILLEGAL Immigrant.

An alternative would be for your girlfriend to visit you.

Stop playing games if you want help.  What country are you from is a simple question.  Elucidating which US Embassy you have to deal with will get you the correct specific help that you need.  There is no need to play the vague guessing game with "a western-friendly country to the US."  Don't turn people off to helping you by being cute.  
 
This is my thoughts to you to help you get quick and accurate answers.  Properly answer questions from people trying to help you.  Don't make them guess with vague answers.

This is quickly becoming something that's none of my business, but what is your fiance's ex girlfriend doing with his phone, or in a position to have access to it? It sounds like something beyond your knowledge is going on here. Either he's way too close with his ex, or he's not being honest with you and cancelled it himself.

Overstaying a visa is certainly against the law. He has not overstayed as of yet, and you just advised them to do so. This is one reason it is so hard for Nigerians to be given a tourist visa in the first place, because of those who come here and do just what you are suggesting. 

Let us suppose she follows your advice and it doesn't work out. Now he loses his tourist visa, is sent home, and banned from entry. I think you should leave the legal advise to the attorneys. 

Ever heard of people lying in order to get what they want? 

Ma'am, my advise would be to WAIT him out. He is here in the US as a visitor, he can visit anytime for up to 6 months as a visitor, continue to enjoy his company and love as you so stated BUT do not make the mistake to marry in a rush. If the love is true it will still be there in a few years. My sister who is younger than me, married a Nigerian about 10 years younger and rushed to be wedded. After the CG arrived, he was gone. The stories about these guys are REAL. ENJOY now but WAIT.