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Australian Standard 1684 Part 2, Residential timber-framed construction, contains the following requirement: Boards over 85mm cover width shall be fixed with a minimum of two face nails at each joist....nails shall be skewed slightly to the vertical in opposing directions....the mechanical fixing shall be supplemented by a minimum 6mm bead of polyurethane or polymer flooring adhesive to the top surface of the joist (Clause 5.5.2.2). The problem with departing from this requirement is that if anything goes wrong, it will be pointed out that you haven't complied with the Australian Standard. Our Technical Design Guide #09 on flooring installation goes into the subject in more detail and may be helpful. A copy can be downloaded from our website free of charge via this link https://www.woodsolutions.com.au/publications.

From your photo it looks as if the cladding has shrunk and split off part of the lap. Glue won't stop timber from moving in response to moisture changes. It just allows stress to build up in the timber until it's relieved by a split or by the joint opening up. So the short answer is that the timber has to be allowed to move. As long as the timber isn't restrained, a lapped joint permits movement without detracting from overall appearance, unlike other joints where gaps caused by movement are more obvious. We assume the cladding isn't fixed to the steel. That can create problems since the two materials react in opposite ways when exposed to heat, ie. steel expands and timber shrinks. A flexible glue would seem a better option than one that sets rock hard, although we haven't researched different types of glue. No doubt adhesive companies could advise on a suitable choice. Alternatively, if practical, nailing allows  timber a bit of wriggle room and the nails can be made a design feature - copper nails etc. as shown here https://www.thewoodworks.com.au/shop/hardware/decorative-nailsExternal link. If the door is in a particularly exposed location you might need to consider a more dimensionally stable timber such as western red cedar.

Unfortunately we have no statistics on the percentage of recycled timber used in new construction. There are one or two technical difficulties. For example, recycled timber often contains contaminants such as nails, grit and so on which can damage equipment, it is not graded to current standards, and obtaining large volumes of the same size members is not easy. Nevertheless, companies that recycle timber do a brisk trade and it is particularly useful in restoration work. You may wish to contact recycling yards to see if they can advise you on their main markets.