Setup: interaction with existing Nginx server?

About the setup procedure at https://docs.gitlab.com/omnibus/development/openshift/README.html

While trying to setup Gitlab on my server, I ran into three issues for which I found no solution in the documentation. Not sure if these are documentation issues or software issues.

1. My server has an existing Nginx webserver setup, running several websites as well as a Gitolite server. The Omnibus Gitlab setup guide mentions a "bundled Nginx webserver", but it does not clarify anywhere how this bundled webserver interacts with an existing installation. I am concerned that installing Gitlab will break the existing installation, or somehow conflict with it. To take away this concern, I need at least some clear documentation of how this works. For example, there cannot be two webservers running at the same port 80 or 443.

2. During the installation (which I tried anyway, after a thorough backup), the home directory of the existing 'git' user was changed without warning. As a result, my existing Gitolite server broke. The existing 'git' user was there for a reason: it was used by Gitolite. I think that at least, the documentation should warn about this, and ideally explain how this problem can be avoided. Also, the installation procedure should not change an existing user without confirmation.

3. During the installation, some errors occurred: "Error executing action create on resource 'letsencrypt_certificate[gitlab.embeetle.com]'". The documentation does not seem to provide any handles on what to do in case of errors; or did I overlook something?