This would work, and let the scripted installation complete without issues. The problem is that now your Discourse installation expects to manage its TLS certificate, which it won’t be able to do once you stop forwarding 80 and 443 to that VM. What you need to do is tell Discourse not to try to obtain a cert or handle TLS at all. And I think (though I haven’t tested this) you can do that by commenting out the web.ssl.template.yml and web.letsencrypt.ssl.template.yml lines in app.yml. Running other websites on the same machine as Discourse Would also have some relevant information, and seems to agree with this.
There’s no real reason to do that, as it’s trivial (if you’re not overly complicating things, as you are) to just get a new cert–the fact that you have an existing cert really doesn’t matter. The complicating factor in your intended setup isn’t that you already have a cert, it’s that this arrangement won’t let Discourse handle its own cert.