Agent Forwarding Through Different Usernames (Git & Deployment)

Simply seeing that ssh-agent is running is insufficient. Look to see if the key you want is present ssh-add -L Have you verified that this key works from some other host? What does ssh-add -L print on that system?

You can also run GIT_TRACE=1 git ls-remote ... and it will print the ssh command it runs. You can manually run the ssh command and get that working before bringing git into the picture.