Neither the speed nor the distance of a falling object is linearly related to time.
The only 'difference' is that they are different categories.
It's like asking "What's the difference between Susie and girl ?"
Or "What's the difference between Cadillac and car ?"
Displacement <em>IS</em> a vector quantity.
Answer:

Explanation:
= Velocity of one lump = 
= Velocity of the other lump = 
m = Mass of each lump = 
The collision is perfectly inelastic as the lumps stick to each other so we have the relation

The velocity of the stuck-together lump just after the collision is
.