The N-color diamond that I was considering was GIA graded. I have bought and owned many, many diamonds ranging from E to J (graded by GIA or AGS), so I know diamond colors. Thus, I have not been brainwashed into thinking that only high color diamonds are the best. As I said before, color is a matter of personal taste, and I am just more color sentive than others.
OP, pay attention to every word in bunnycat's post (#10). Make sure the center diamond speaks to you, and you are Ok with its brilliancy and color. Check to see if the diamond is eye-clean, and any visible marks are acceptable to you. Honestly, I am worried about two things: the diamond's (cut) appearance in your photo and AIGL grading. Although photos alone cannot be 100% trusted to declare the diamond good, i do not feel comfortable with the appearance of your diamond in the photo posted (I know I am risking a lot of protests for speaking my mind here, but I am worried for you). For comparison, I can tell from photos that jadie1's and bunncat's diamonds are very, very nice in terms of cut. As for the grading, I do not feel comfortable with grading houses other than GIA and AGS for reasons already stated by bunnycat. First, the actual color can be several grades lower if graded by GIA. Second, the graded clarity is already SI1 (which has a good probability of having visible inclusions), the actual clarity if graded by GIA or AGS may be lower further, increasing the chance of visible, large inclusions (if ending up being I1-2), which may impact the value of the diamond more than expected. If an important part of your motivation for considering this diamond was price, then for more reasons you need to confirm its true value with your own jeweler (hopefully no conflict of interest) and/or independent appraiser.
The one great relief is that you are able to return the diamond if desired.
P/S - if you plan to spend money to change the setting, then include this cost of not using the old setting in your calculation of the worth of the center diamond. Doubt very much you can get any money for the unused setting. Melees are not worth much, and the setting is basically scrap metal in terms of resale.
Do you mean that you plan to remove the center diamond from its setting for evaluation? If so, then first make sure that by doing so will not null your return policy. Sometimes, the prongs may be weakened or broken in the removal process.