When you clone the build environment, you are cloning the the build environment for all the OE-A images. All the OE-a images can be made from that directory. I'd ignore the instruction in the making openatv 7.2, vix 6.3 directories and use the branch name that you are forking. e.g "OE-A_5.2".
The adduser part has always given me grief with having to rights etc. Probably have to do something that is not written down and is meant to be common knowledge. Again, openvixbuilder, openatvbuilder or any other name can build any image in the OE-A team.