Lars Krutak has written a fair bit about tattooing in Africa. I believe he has written books about this, but here are some things available online:
Tattooing in North Africa, The Middle East and Balkans by Lars Krutak
Tattoos of Sub-Saharan Africa by Lars Krutak
TRIBAL TATTOOS of MOZAMBIQUE'S MAKONDE - by LARS KRUTAK
I once worked with a woman from Senegal who was casually interested in tattoos are she talked a little bit about traditional tattooing in Senegal, nothing really in depth, but there is a tradition of tattooing there.
In general, if you want good information, I'd spend less time on the Internet and more time talking with people. I would guess that Lars Krutak, who wrote the above articles, has an email address and probably does speaking engagements or lectures. You said that your nan doesn't like tattoos, but what about your other relatives? They may know things, and even if you don't get anything useful about tattoos in Ghana, maybe you'll get some good stories regardless. You're in the UK, right? I'd also suggest talking to guys like Curly, Alex Binnie, or Tomas Tomas because I can only assume those guys have a tremendous amount of knowledge and information about pre-technological tattooing, and I'm sure they'd be super happy to do a tattoo like this. Finally, you have access to some of the world's greatest libraries and universities, use those.
if the shading is there to cover something else up... is the shading preferable to whatever its covering? Not sure I have any answers for you, Looks like the artists went pretty hard on the edges of the owl. Maybe some color will make it pop some more, but I have a tattoo on my foot and it has faded a lot on its own and I never went back to it, I just don't think ink holds well on the foot IMHO. I'd leave it alone and move on.