How to List Tags in an Ansible Playbook

IMHO there is no such variable to "see which tags are set in a playbook". There are only ansible_run_tags and ansible_skip_tags. I'm afraid the only option would be to write an ansible-playbook wrapper to check if the supplied tags are present in the playbook ("to run in check-mode first, and manually parse the output").

BTW. Below is a simplified list of completed roles

completed_roles: "{{ completed_roles|default('') }} {{ this_role }}"

To avoid "pre-tasks get run, as do post tasks, which makes it appear that something is happening" you might want to use import_roles and when condition instead of tags. Below is an example where role3.yml and role4.yml print a message only.

play.yml

- hosts:
    - localhost
  tasks:
    - import_role: name=role3
      when: selector|default('') in [ 'role3', 'all_roles' ]
    - import_role: name=role4
      when: selector|default('') in [ 'role4', 'all_roles' ]

.

> ansible-playbook -e selector=role4 play.yml | grep msg
    "msg": "role4"

.

> ansible-playbook -e selector=all_roles play.yml | grep msg
    "msg": "role3"
    "msg": "role4"

A value you set with set_fact will be available between different plays. Keep in mind that set_fact are set for a specific host. Your first play is run against localhost so the fact is part of the localhosts variables. So it the following play you should be able to access it with a task like this.

- debug:
    var: hostvars['localhost']['hostmap']