Sha,

Trees do not normally have the support shown in your diagram. Commonly the tree is supported on the conductor (outer pipestring) flange and other casing hangers are supported by it, via the tree.

Downward movement can occur ... but it is rare and usually not of any great magnitude compared to the usual upward growth. Downward movement in my experience is limited to (water) injection wells, where the injected fluid is significantly lower temerature than the ambient strata temperature in which the well was initially drilled and cemented. This causes a thermal contraction of the wellstring. It may occur in dual producer / injection wells too.