Question 1: Can the velocity of a body reverse direction when acceleration is constant? If you think so give an example.

Answer

Yes, this is quite possible for a body to reverse its direction when the acceleration of the motion is constant.

Example

Throw a ball vertically upward with a certain velocity v_i. The motion is an accelerated one. The gravitational force acting on the body attracts it toward the center of the earth. We know that this acceleration is g = -9.8 ms^-2.
Apply the equation of motion,

Put the value of the acceleration,

Initially, the value of v_i is greater than the term (-gt) and the body moves up. However, with the passage of time, when ‘t’ increases the term ‘gt’ also increases. At a certain time v_i and ‘gt’ will be equal and v_f = 0. This would be the highest point of the motion.

Earth still attracts the body with the same force and acceleration. After this moment, the body changes direction (reverses) and now moves downward with the same acceleration.
Hence, it is possible for a body to reverse its direction when the acceleration is constant.

Consider a spring-mass system (mass attached with a spring at one end). Let the spring is fixed at one end and the surface at which the system is placed is ideally friction-less. Now you pull the mass to some distance ‘x’ and then release. The mass starts to and fro motion on the friction-less surface. How can you say this is another case of a body reversing direction at constant acceleration?