Silent Hill Heaven

I read that in order to unlock Cynthia's alternate costume, one must collect Eileen's nurse costume and unlock all 4 endings. Do I have to start a new fear file for each playthrough or can I save time and just reboot the game from different parts of my original playthrough to unlock all 4 endings?

On a related note, I also read that to unlock the machine gun, I need to get 9 stars. There are two sets of stars on my results screen, the bottom one has exactly 9 stars though, I think the top had 6, do I qualify?

I can't speak to the costume part of this, but I assume the 9 stars refers to a "90%."
Going off your post, it sounds like you achieved a "69%" score.

I think you can just load an old file and aim for a different ending. If memory serves that' s what I did. I' m not 100% sure, but checking for yourself if that works shouldn' t take long (the end game stats screen tells you how many endings are considered obtained).

If you kept a good + pre-walter file then getting all four endings from that single file is easy and quick.

And yeah, sounds like your score was 6,9.