I don't think calling session_start after output is an issue here, it would only be an issue if a session wasn't already setup and in your cookies. And as long as your not destroying the session anywhere, you should be able to do what you are trying to do.

You are better off using the database for data instead of the session, I've worked projects where the session gets very easily corrupted due to constantly adding/editing the session - it is just not really cut out for heavy exercises.

It is so easy for 2 scripts to clash if they are both accessing a session and trying to write to it at the same time, a database is literally perfect for storing, fetching and constantly modifying data. So use what is built for the task and forget trying to use sessions.