Flags, rules and node type situation
It sounds to me that you should have a look at the Entity Registration module as an alternative to simplify your current setup. Some more details about it, from its Community documentation:

The Entity Registration module provides the ability for users to register for any entity in Drupal. Registrations are fieldable entities and can collect any information you need from your registrants. By attaching a Registration field to any entity, you enable users to fill out a form containing customized fields to register for that entity.
Entity Registration currently has the following features:

Registrations are fieldable entities, so collect any information you need from your registrants
Unlimited types of registrations, or bundles.
Enable registrations of a given type/bundle on any entity type or individual entity through a registration field.
Per entity registration settings like capacity, reminders, etc.
Send broadcast emails to registrants.
Allow a single user to register multiple registrants.
Integration with Views and Rules via Entity API.