We don't generally get involved in individual design projects, but we have a Technical Design Manual that will answer some of your questions. It's titled Domestic Timber Deck Design and can be downloaded free of charge from our website via this link https://www.woodsolutions.com.au/publications.

We understand that sawn timber which is clean and untreated (ie. not preservative treated) is readily recyclable into mulch, animal bedding, particleboard or fuel. However, you have to enquire with local facilities if they will also accept plywood for recycling into any of those applications. It varies between facilities. Most plywood made in Australia is made with either phenol-based resins (A-bond) or melamine urea-formaldehyde (B-bond) that are very stable and strong.

From your description and the image attached to your message the most obvious type of timber would be the group of Peltogyne species commonly known as "purpleheart". You will find more information about purpleheart on the Fine Woodworking website via this link https://www.finewoodworking.com/forum/where-is-the-purple-in-my-purple-heart. It is a South American species not widely stocked in Australia but available from specialist timber suppliers. 

In our opinion the timber should be suitable to remain in service. Wood rot is unlikely in timber treated with creosote, although we weren't sure whether it was a pressure treatment or a brush-on application. In any event, once timber is fully dry (and kept dry) decay fungi cannot attack timber.To determine when it is fully dry, testing with a moisture meter would be helpful. The aim would be to bring the moisture content down to 12% or below. This should be achieved before new wall linings are installed.