Radiata pine isn’t prone to checking under all circumstances. For example, kiln-dried pine in a stable environment, such as pine furniture kept indoors, isn’t likely to check. Checking occurs when pine (and most other timbers) are exposed to rapid drying while they have a high moisture content. As timber dries it shrinks.

Generally speaking you seem to be on the right track, but you really need someone to go through the construction drawings in detail, or perhaps visit the site, since it's an existing building that you are modifying. It's hard for us to get a full appreciation of all the factors involved via email. We also point out that we have no power to give building approval and it will be necessary sooner or later for you to engage a suitably qualified person to advise you. If your usual certifier is "too busy" we suggest you consult a fire engineer.