(1) the relationship between a and c can be known from eccentricity, and b= 1 can be known from the length of short axis, and A =b +c is added to get a=2.
(2) From the equation of straight line MD and elliptic equation, we can find the coordinates of m points, and by the same token, we can find the coordinates of n points, and then we can find the length of MN.
In this way, the equation of straight line MN can be obtained, and then the distance d from point D to straight line MN can be obtained.
Therefore, s can be represented by k.
The value range of k can be obtained by solving inequalities.
Note that the range of k found here should intersect with the range of k when the above equation has a solution.