Initial height of tower, h0 = 50 m

At maximum height, the final velocity $v = 0 \text{ m/s}$. We use the kinematic equation $v^2 = u^2 + 2as$.
Here, $a = -g$ because gravity acts downwards, opposite to the initial upward motion.

This is the height reached above the tower.

The maximum height from the ground is the initial height of the tower plus the height gained above the tower.

The maximum height reached from the ground is $\boxed{\text{70 m}}$.

We use the kinematic equation $v = u + at$.
Here, $v = 0 \text{ m/s}$, $u = 20 \text{ m/s}$, and $a = -g = -10 \text{ m/s}^2$.

The time it takes to reach the maximum height is $\boxed{\text{2 s}}$.

This is the height gained above the tower, which was calculated in part (i).
Distance upward $= 20 \text{ m}$.

The stone falls from its maximum height (70 m from the ground) to the ground.
Distance downward $= H_{max} = 70 \text{ m}$.

Total distance = Distance upward + Distance downward
Total distance $= 20 \text{ m} + 70 \text{ m} = 90 \text{ m}$
The total distance covered is $\boxed{\text{90 m}}$.