While there are limits to the bending that a flange connection can take, I've typically treated them as rigid.

Now, if you have access to Nozzle Pro, getting nozzle stiffness values is easier than falling out of bed. You pretty much just have to fill out the basic dimensional information on the main screen, fill in the pressure on the screen that comes up from the LOADS button. Then run it.

I typically use uncorroded wall thickness for the shell and nozzle when checking for stiffness values for Caesar. I'll then use the corroded wall thickness values when checking the forces/moments that Caesar gives me to validate the nozzle stresses.