My answers to blendshapes.

1. Yes. The number of vertices must be the same because if the base shape is trying to change into the target shape and the vertices are not equal, some vertex have nowhere to go and the model might look silly.

2. No. Blend shapes can be used for just about anything so long as there is a replica piece with the same number as the base model's vertex that is needed to be changed.

3. Because if the model can only be changed to one type of angry expression. If the animator wants a sightly different angry expression, it is going to be very hard for the animator to achieve a slightly different angry expression. What can be done is that the angry expression can be broken down so that one part of the angry expression has its own replica.