1. Not every item has to be good all the time. The cocktail is an item that is good under certain circumstances but not others. If you see a new item you haven't encountered before in the rotation, you can check the paclopedia from the pause menu to read what it does before you pick it up. This is a game that is meant to be played and replayed and so you will ideally learn things as you go along.
2. Difficulty balancing is one of those things I will have to continue working on as I improve the game. I think I'm making progress on that front, so stay tuned!
3. The tall grass obscures dots and items as part of its behavior, it's a gimmick specifically meant to be obtrusive. Even then, grass which is "concealing" items will be parted and darker. I can't speak on the flowers in Jungly Steps, I have never heard of anybody else struggling with that.
4. Trog is a playable character you can pick in the player options menu, not an enemy. Just press the button with Pac-Man on it and you can swap to him if you want.