Your answer to (a) looks fine.
(b) I get the time of intial contact to be 0.43 s (when v = 4 m/s) and the time when the ball leaves the surface (speed starts to decrease) to be 0.58 s (when v = -3.6 m/s).
Then where m/s.
(c) . Get from (b). s.
Scale reading uncertainty: You should assume all my values for t are and my values for v are ........