It is possible to add 3D colliders and Rigidbody2D (or 2D colliders and Rigidbody) from script

2) How can we reproduce it using the example you attached
1 Create new project
2 Create cube
3 Create script A that has

gameObject.AddComponent<BoxCollider>();
gameObject.AddComponent<Rigidbody2D>();

4 Add A to cube and press Play - No error, Collider is not removed
(collider should be removed or should give exeption)

Same for
gameObject.AddComponent<BoxCollider2D>();
gameObject.AddComponent<Rigidbody>();