Oh wow, thank you so much for your feedback! It all helps me improve in the future so definitely not taken personal! The time you've taken to enjoy my game means a lot to me.
Some of the things you mentioned I really wanted to implement myself but was limited by resources sadly. Especially the feedback concerning the enemy AI and music.
Others I really understand what you mean! I'll take those notes when I work on my next project :)
The lighting in the marsh is unfortunate, the first level in that area is much darker than the rest of the game.
As far as not being able to progress in those sections you need to crouch when near (and sneaking past) the enemies by pressing C.
You can also level select in the main menu if you get too stuck!