Answer to Question 1
The urethane bond is easily formed with little consequence of the groups that are between the active groups on the polymers. Therefore, a wide variety of structural shapes are possible for these groups. These groups determine the basic properties of the polyurethane and, therefore, give rise to the variability.
Answer to Question 2
One method would be to add a rubber toughener to the epoxy resin. Another method would be to change the nature of the epoxy so that it is more aliphatic and less aromatic.