The reply is certainly not disappointing, you are looking for an easy answer. Using CII is always going to be an approximation for what happens in the field, you should always consider the option of re-routing if it means you can substantially reduce stresses.

Yes, two different types of material can be welded, that is a fabrication issue solved by weld procedures, however you need to be consulting your welding engineer for that query.

As you are located in Australia, most companies there have very good experience and you should definitely be asking these questions to your supervisors or colleagues, as you will get prompt and appropriate responses.