We agree, it's very difficult to maintain a complete seal. Even if the bearers are pre-coated all round, the coating may be damaged during installation. Also movement in the timber could break the seal and create an opening for water to enter. It's best to leave the timber uncoated so it can dry out as quickly as possible when the water level drops. Note that all fasteners will need to be stainless steel. You don't mention the type of timber. Preservative treatments have limited penetration into hardwoods and can only be relied on to protect the sapwood.

If the stringers are fixed to wall studs both sides, essentially the stringer span becomes the distance between studs, measured on the angle. So the short answer is yes, the stringer size could be reduced, consistent with having enough width to accommodate the treads. However you would also need to be sure that the stud walls can accommodate the extra load imposed by the stairs, ie. the walls will be taking more load than they would be if the stringers were spanning the full 4.3m without being fixed to the studs.

It's important for your clients to understand that the shrinkage figures quoted on our website for spotted gum (tangential 6.1%, and radial 4.3%) refer to the shrinkage that the timber undergoes in drying from the unseasoned condition down to a moisture content of 12%. No doubt the decking boards used for the screen will be kiln-dried, ie. 'seasoned', so their moisture content at the time of installation will already be around 12%, in which case shrinkage will be minimal unless there is a long period of hot dry weather.

AS 3959-2009 is now superseded by AS 3959-2018, but the focus is still on preventing fire from entering the building envelope, rather than on preventing its spread internally. So there are no restrictions on internal wall construction, nor on the internal face of an external wall. Regarding FRL's associated with external walls, in BAL-FZ one of the options for a timber-framed external wall is to utilise a system with an FRL of -/30/30 when tested from the outside. Such a system would also satisfy external wall requirements in a lower BAL (clause 3.4).

Our Technical Design Guide no. 21 titled Domestic Timber Deck Design will be a useful reference,  available for download free of charge via this link https://www.woodsolutions.com.au/publications. The three timber species you mention are all 'durable' in both senses of the term - ie. wear resistance and rot resistance. All three fall into Durability Class 1 in terms of AS 5604, Timber - Natural durability ratings when placed in above-ground situations.

Oriented Strand Lumber (OSL), more commonly termed Oriented Strand Board (OSB), is not marketed as a structural product and hence doesn't have a stress grade according to the Australian system. Strength properties are contained in a US document available via this link https://www.pfsteco.com/techtips/pdf/tt_osbdesigncapacities.