Hey thanks for playing! To answer your questions:
The collectable items you find around the levels aren't kept track of by the game, so effectively they're useless haha. The idea for 'useless collectables' came from a game called FREEDOM: Diegesis which I highly recommend checking out.
The double jump is meant to work like that. It's sort of to stop players from being able to jump too high lol
And there's just the one ending, sounds like you reached it 🙂 glad you enjoyed!