Thank you for taking the time to play our game and give such an in depth review. We agree the game is a bit difficult, but you're right that we didn't have a great way of testing it. We tested it with a few friends, but most of them are very into the FPS genre, so they probably weren't the best choices.
Our goal way to make the enemies feel swarmy at the start and very strong at the end and I think the levels were built in a way that reflects that. I think the problem comes from the inability to do more damage by hitting headshots, which was a thing we planned but didn't have the time to implement. It's very true that the solid FPS feeling is quite ambitious.
As with the tutorial, we needed a way of displaying a story to the player, but we didn't really know how and just settled on doing it in the tutorial. If we had more time we would've made a separate category for the handbook, which would contain a tutorial page with just the basics, an enemies page, a weapon page and a lore page.