Thank you for this detailed, yet spoiler-free review! I'm glad you got this much out of the game and didn't quit after reaching the first ending haha Thank you for buying the extended edition, but please know that the main game is the same as the standard edition aside from two more endings you can get pretty early on. The main difference is the bonus room with some more after-scenes and art and stuff. :) Hope you have fun exploring these things, though!
For the benefit of future readers debating whether or not to go for the extended edition - I recommend it!
As Hatoge said, the extra endings are accessed quite quickly, but one of them answered a "Yeah but why doesn't he just--" question I had, so that was a plus! Also the bonus content is fantastic; if you've grown to love the cast the extra details about them are a wonderful addition, and I love seeing a bit of history/background into the development of projects too. I also really appreciated the after-endings, they added to the story without feeling like a fanservicey cop-out~