A Class 5 building is generally an office building, but you don’t mention how many storeys, or where in the building the wall will be. The BCA requires buildings (except for Classes 1 and 10) to be constructed according to Types A, B or C construction. Type A is the most fire-resistant form of construction while Type C is the least. The type of construction is determined on the basis of the rise in storeys.

One website quotes a weight of 1.5 kg/m² for Yellow Tongue flooring, but this is incorrect by a factor of about 10, so some care is needed taking data from the net.

Much of the information you seek about Boral hardwood products is set out in company literature. For example, the Boral Product Guide available here https://www.boral.com.au/sites/default/files/media/field_document/13369_Timber_ProdGuide_FA_LR.pdf advises that cladding should be fixed to studs at 'maximum 600 mm centres'. Boral anticipate that their cladding will be fixed with nails (Table 5).

Merbau is inclined to 'bleed' but this doesn't happen spontaneously - what it means is that a brown tannin stain will wash out of any bare surfaces exposed to rain or other moisture source.

No doubt your consultants are referring to Part 5 of Specification C1.1 in NCC 2019. Clause 5.1(e) calls for a floor separating storeys in a Class 9b building to have an FRL of "at least 30/30/30", or a protective covering on the underside if the floor is combustible. It appears that your floor system will have 19mm plywood on the underside. You could look for a fire retardant coating for the plywood that would provide a 30/30/30 FRL.