I like how they explain that he must damage the kind of enemy at least once before being able to go all in. This limit the possibility to just throw something once and stop. Still he should pick up a ranged weapon, an arrow or something else would probably have more chance to pierce over the enemy armour (at least more than throwing a random rock or bone), and become a valid target.
How not bro? he did hit it while conscious with a pickaxe before activating the skill.
Auto collect that is op, he can collect anything he never collected before.
Yes, he perfored the helm and skull with the pickaxe.
While not explicitly stated, I think the Auto Collect skill also works in a similar way. The MC must have seen/handled/gathered the material before. I understood it this way by
chapter 3, page 16, where he says the available ores are more after he meet new ones, in that case different sizes of glowstone.
For the place where MC studied all those rare materials I would say in his training for Special Force. We see in a panel in
chapter 12, page 16 the MC handling the "worth as much of a castle"-plant, and being others there. The team had enough money to probably gather basically every kind of materials.