How can we account for Sleeve/Fitting material? Does the analysis assumes the same composition for both header and fitting? Is so, is accounting for the difference as simple as inputting the composition with the highest CE value?

The current model (V 4.2.2) doesnt allow predicting hardness of two different materials. The inability to predict hardness of two different materials is an issue with the current model.

What is highlighted in the question is something that many others do, in that they pick the highest CE material and evaluate that to be conservative.

Another approach that is widely used was to evaluate the pipe material using the flow conditions and then evaluate the fitting material with something like 1/2 the flow rate. The reasoning is that the sleeve HAZ cooling rate is not as effected by the flowing product as is the pipe HAZ.