The cannon is much heavier than the cannonball, so it has the same momentum, but not the same velocity. The cannonball has less mass to overcome, so it accelerates more quickly.
Positive acceleration is a parabola pointing upwards.
Negative acceleration is a parabola pointing downwards.
I am not sure what you mean by the balanced forces. But I guess it means no acceleration which is a straight line. It can be a horizontal line or an inclined line.
Moles Pbl2 = 0.8628 g : 461.01 g/mol = 0.001871
moles I = 2 x 0.001871 = 0.003742
[I-] = 0.003742/ 0.0429
= 0.0872 M