SIPS Part B Fire

[JuddlyFire]

I have to hold my hands up SGVB. I don't have enough experience to be definitive.
Your initial query/concern hasn't been satisfied or clarified. If anything only further confused. I'm sorry.
For the record:

The entire unprotected area aspect in relation to cladding does not make a lot of sense to me as the classic unprotected areas of a building (windows and doors) are surely primarily a risk to another building (on the other side of the boundary) if fire started INSIDE (I am sure most fires start inside a building) ? And we know the internal walls meet the requirement.

What if the fire originates in/on/from the adjacent building?

So why scrutinise the cladding on the outside of the building in Section 5 that is all about internal fire

Section 5: Internal fire spread – dwelling houses
Loadbearing elements of structure
Fire resistance standard

5.1 Elements of structure such as structural frames, beams, columns, loadbearing walls (internal and external),

Your original post raises a singular self contained question: "The question is now raised about the resistance to fire of the structure itself."
We have explored the regs trying to decipher what must meet which standard internal, external and structural but it's all whistling into the wind unless we know what aspect/s specifically the Building Inspector is concerned with.
You say "The outside of the building is clad in a combination of corten steel and timber. Building Inspector is happy it meets the surface spread of fire under the regs." and "Internally, it is simple (pink board etc) and the inspector is happy..."
So:

The issue is focused around B19 Fire Resitance

A, B or C?
I think we can rule out B (which would also put a lot of questions to rest) but A & C are distinctively different. If his issues are with either or both 'Resistance to collapse' and 'Resistance to the transfer of excessive heat' it closes multiple lines of enquiry. While opening others.