Found someone who scanned the first 2(I think)chapters of Zenki into spanish.
http://www.freewebs.com/zenkimanga/
The only process unfortunately is a male demon who I assume is Zenki the main male character going through AR.
Bright side, now we know there 12 volumes of the manga and since the anime has 51 episodes and an OVA episode. So the chances of the anime covering all the manga is relatively high.
Can you post the 7th volume of Zenki you found. Please take your time looking for the rest.
In the meanwhile I found a second hand store that sells old raw manga. I saw a few volumes of GS mikami there.
I`ll see whether they have other volumes of Zenki.