1a) Is your modeling correct, with the rotation set to zero? Most likely "yes". Since you are at a nozzle, you can't leave the rotations "free" (unspecified). There shouldn't be any rotation due to the expansion of the turbine, but only the vendor can answer that one for sure.

1b) Is there a better method so that the stiffnesses can be relaxed? Again check with the vendor. Perhaps he can give you nozzle stiffnesses.

1c) Is WRC applicable? Maybe, it depends on the various geometric ratios. If you exceed the limitations of WRC you're better off using the displacements and being conservative. There are a number of posts in this forum on WRC limitations. Use the [search] option above to locate and review these.

2) Are the load cases correct? Yes your load cases are correct for the "cold" and "hot" cases. However, you should also check the "hot seismic" event. You can do this with:

L1 + L3 (algebraic)
L1 - L3 (algebraic)

3) Is there any force/moment leeway during the seismic event? Again, you need to get this sort of information from the vendor.