I will try to answer the questions that I know.
1) You can refer to this example . PDE app is used in this case to find plane stress and strain. However, you can do it using PDE toolbox command line approach as well.
2) I am not sure what you mean by in plane displacement at boundary.
3) Looking at the way the problem is formulated, res.NodalSolution consists of solution to [u1, u2] where u1 = w u2 = \Delta^2 w = v
and XGradient and YGradient are gradients in x and y direction respectively for u1 and u2.
I hope this answers your queries.