Regarding variable precedence, it's pretty simple to grok in 2.x+, and well-documented now (it was a serious nightmare, and a very valid complaint... but isn't as much anymore).

For sharing common group_vars, what I do is my inventory passes an environment-aware variable which I include in the playbook by doing like:

`- include: "env_vars/{{ environment }}/main.yml"`

I feel I sidestepped some of the AWS modules issues simply by using Terraform. I still build all my images in Packer using the Ansible builder though.