Q1.
Can you explain now why the magnet has no effect on Sample B?
Answer
Because there is no iron left in Sample B as iron. On heating, the iron and the sulfur combined chemically to form a new substance — iron sulfide — and a compound has properties completely different from those of the elements that formed it.
Iron + Sulfur (on heating) → Iron sulfide
Why it happens: Being attracted by a magnet belongs to iron, not to iron sulfide. In Sample A the iron particles were only mixed with sulfur, so they kept that property and the magnet could pull them out. In Sample B the iron particles are chemically bound to sulfur particles in a fixed ratio and are no longer free; the compound they form is a different substance with its own set of properties, and magnetic attraction is not one of them. This is also why the iron and sulfur can no longer be separated from Sample B by any physical means.