The CB will curve around the OB on a no spin cut shot. Please allow me to elaborate. From a physics perspective, the newtonian law of action and reaction applies. When the CB strikes the OB, the OB responds with an equal and opposite force. But the CB still has forward momentum and the result of it wanting to travel forward and being pushed to the side causes it to curve. Please refer to the short video below for further explanation and slow motion evidence.
In spite of our disagreements, this is an useful thread.
https://youtu.be/heaU2bmW5n4
Julian
In the video Barry Stark is playing the cue ball with full follow. That is why the cue ball curves forward. Your stated reason for the curve shows a misunderstanding of what is going on. Barry does a very poor job of explaining what is going on from a technical perspective.
As for the the initial path of the cue ball being forward or back of the 90-degree line, there are many reasons why that may happen:
1. The balls are not perfectly elastic. If they loose energy on contact, Newton's laws of motion require the angle to be less than 90 degrees. That means that the cue ball will go forward of the perpendicular to the object ball's path. No ball is perfectly elastic. Ivory balls are good to demonstrate this because they are considerably less elastic than cast phenolic balls.
2. There is throw. Throw will change the path of the object ball without changing the outbound direction of the cue ball. If throw is the only non-ideal thing going on, it can make the angle either more or less than 90 degrees.
3. The masses of the balls are not equal. This happens most often with old, worn cue balls which will tend to bounce back from a heavier object ball and make the angle more than 90. If you change that to a new cue ball, it will probably be a little heavier than the object balls and go forward of 90.
4. The cue ball is in the air at contact. This will make the cue ball go forward of 90. The cue ball is in the air for part of nearly every shot. Shoot harder and it is airborne longer.
As for Earl's shot, items 1 and 4 would have taken the cue ball forward while 2 on that shot would have tended to make the angle wider than 90 (equivalent of pushing the cue ball back).
The tangent line is an approximation, it will not travel exactly on the tangent depending on speed and spin. If you want to get technical, the CB does not travel straight on *any* cut shot, it curves right after impact. 
The tangent line is an approximation, it will not travel exactly on the tangent depending on speed and spin. If you want to get technical, the CB does not travel straight on *any* cut shot, it curves right after impact.
