psql: FATAL: role "postgres" does not exist
Not a full answer, but consider the manual about initdb here:

-Uusername
--username=username


  Selects the user name of the database superuser. This defaults to the
  name of the effective user running initdb. It is really not important
  what the superuser's name is, but one might choose to keep the
  customary name postgres, even if the operating system user's name is different.


Meaning ...


When running initdb as OS user postgres, the initial DB superuser role is postgres as well - in standard Postgres. I have heard of systems bending this rule ...
You should be able to force the name postgres with --username=postgres for the initial superuser - unless your package seriously messed with all this.