Error Text: What does it mean?

All of those are “normal”, but annoying.

As soon as you placed that stockpile, Hearthlings got tasks to store items. Hearthlings don’t “think” about where they are storing and item once they start. They only think about where it goes again once it is on the ground. To avoid this, you can change your gameplay settings to default to all filters off.

Basically the same as above. Hearthlings get assigned tasks, (like mining), and will complete them unless they can’t. So, those 3 Hearthlings were always assigned to mine sections of the of the area, just not in the most efficient way. To get a better indication that another Hearthling is coming, you can turn on Heartling paths via gameplay settings.

Standing in the wrong spot is just by chance. The pathfinder isn’t intentionally having them stand there. If you are looking for more stockpile filter, check out @Shinsaka’s excellent Better Stockpiles Mod.