Hello
1. if you look at the machines in front of the portals you can see what you can collect.
If you want to acces the hidden level you must disable easy mode and find the hidden level in, HUB 1 Temple, HUB 2 Castle, HUB 3 Alien.
These levels are really tough since its filled with traps (and checkpoints so you will die a lot) so pack up on lives if you attempt these.
2. Just hidden levels or farm up gems to buy the gun in the shop, and you can try for death_coins because they will also give you a pack so you can carry more ammo.
3. HUB 2 Temple level up the shortcut kill the minotaur, Spooky 1 for early farming, or City Level 2 for late game farming (the fights)
You ranting is a good thing because the game interests you, if i get more bug reports I update the game and maybe tweak some of the game, no promises :P
And later as I took a long break ill make something more, I am having a few idea's.
Good luck with the challenges.