Thanks a lot for working on this! Sadly did I never use Heroku so I can not tell you what exactly is going wrong. Expect, however, that the issue is related that it starts on a different port than you expect. n8n runs by default on port 5678 but you do not mention it anywhere in your guide. So do not know how Heroku would know about that and I would say it can not work.
If that is not the problem or something else is still not working, it is maybe a good idea to check out the following documentation and guides. Maybe reading through them helps you figure out what could be missing or is wrong: https://docs.n8n.io/#/configuration