This is a question that should be put to the Standards Australia Technical Committee responsible for AS 1684.2:2021. The same wording appeared in AS 1684.2:2010 but we don't have data on the basis for the clause. Lintel trimmers, of course, are different from windowsill trimmers in that they act in conjunction with the lintel. Design deflections of lintels in combination with lintel trimmers have been determined to maintain clearances between the house frame and the window or door frame. Windowsill trimmers act alone.

It's very difficult to estimate the life of a western red cedar shingle-clad wall in the Melbourne climate, since it depends on weather exposure (whether there is any shielding), roof overhangs if any, maintenance of a finish and similar factors. However, some general observations might help. Timber-clad walls survive best when water runs off without being trapped. Western red cedar is a durable species and is rated Class 2 outdoors, above ground, according to Australian Standard 5604:2022, Timber - Natural durability ratings.

WoodSolutions is purely an advisory body and we don't produce timber products, so we would not have a use for your blackwood. Its marketability will depend on a number of factors including the quality of the logs, for example whether the trees were pruned to produce high quality 'clear' (knot-free) wood. Other factors include whether you are located near a sawmill, preferably one with kiln-drying facilities, how many trees you have, accessability of the site to harvest the logs and so on.