$1)$ if one should be intractable how the other should be intractable can you justify this?

$2)$ if one should be intractable other should not be intractable how is it not possible

$3)$ i need what is the difference between polynomial time reduction and planned direction is there is any diiference

$4)$ give me the relationship between $3-$sat, independent set, vertex cover and set cover in both polynomial time reduction and $$planned$$ direction of the polynomial time reduction?