The damage shown in your photos doesn't appear to be fungal decay (rot). It's difficult to see exactly what is going on, but decay would present as brown cubical material ('brown rot'), or whitish stringy damage ('white rot'), rather than flakes of sound wood interspersed with damaged material. It may be an advanced case of Lyctus borer which produces fine dust similar to talc powder. This could be the 'powder-like buildup' described in the caption to your image #3.

Your blackbutt boards are relatively wide. Movement lengthwise is negligible and can be ignored, but timber movement across the grain is proportional to width. Movement can occur if the timber changes its moisture content in situ. In an air-conditioned environment in Melbourne timber products have a moisture content ranging from 7-10% with an average of 9% (CSIRO Forest Products Newsletter No. 357). Blackbutt has a "unit movement" factor of 0.37% per 1% change in moisture content, according to AS 1684.2.

In answer to your questions, 1. Australian Sustainable Hardwoods (ASH) manufactures in Victoria at Heyfield and focuses on sustainability, more info on their website here https://www.responsiblewood.org.au/meet-your-certified-supplier-this-is-ash/. Other companies may also produce flooring in Victoria but we have not conducted a thorough search. 2. We understand that "Sapphire Blonde" and "Mixed Blonde" are marketing terms for mixed parcels of species of similar colour.

Spotted gum is less likely to leach tannin than many other hardwoods, but not totally free of tannin. Queensland's Department of Primary Industries puts it this way: "Has lower tannin than most other eucalypts, therefore staining of paintwork, brickwork etc., as a result of water running over unpainted timber surfaces is unlikely to occur". Note that the reference is to "unpainted timber".