Your best source of information is a Technical Data Sheet titled Timber Decks - Commercial, Industrial & Marine. It's number 7 in a series published by Timber Queensland, and can be downloaded via this link http://www.dtmtimber.com.au/downloads/TQ_TDS/tds_07_Timber-Deck_final.pdf. Table 4 gives decking board spans under different load conditions. Spans vary according to the stress grade of the timber.

The main aim of AS 3959, Construction of buildings in bushfire-prone areas, is to prevent fire from entering the building from an external source. We consider it is the fire resistance of the wall in total, rather than the outer skin, that must resist fire in BAL-FZ areas. In the case you describe it seems the required fire resistance level (FRL) of -/30/30 would be provided by the core-filled blockwork, with hardwood cladding as a decorative overlay.

The photo that accompanied your message looks very like iron staining. A data sheet published by the US Department of Agriculture explains the problem in more detail. A copy can be downloaded via this link https://www.fs.usda.gov/research/treesearch/48449. It's sometimes difficult to pin down the source of the metal contaminant, but if there was any metal work nearby or overhead (grinding metal balustrades, cutting eaves gutters, etc.) such procedures could have produced metal dust or particles.

Transparent wood (thin sheets of wood infused with transparent epoxy) has been produced by US researchers, and is described on several websites. The product is lighter than glass, has better insulating properties, and is more resistant to accidental breakage. However, we are not aware of its availability in Australia, or its commercial production anywhere else, being still at an experimental stage at present.