This question is off-base and misleading from the beginning.
When you jump off the Earth, your momentum changes, <em>and the Earth moves away from you with an equal change of momentum in the opposite direction</em>.
1). Momentum is conserved when you jump. But we don't feel the Earth moving. Since the Earth's mass is a bazillion times greater than YOUR mass, the speed with which the Earth moves away from you is only one bazillionth of your speed. That way, the product of (mass) x (speed) is the SAME for you and for the Earth, and momentum is conserved.
2). <em>Of course !</em> If everyone jumped at the same time, the Earth's momentum would change. In answer-(1), I explained that the Earth's momentum changes whenever <em>ONE PERSON</em> jumps. So 7 billion people all jumping at the same time would certainly make it change.