The probable service life of F14 unseasoned hardwood will depend on the species of timber, so it's hard to draw a comparison with H3 treated pine without knowing the hardwood's species and Durability Class. Using seasoned hardwood instead of unseasoned hardwood would eliminate, or at least greatly reduce, problems of twisting but at greater cost and reduced workability. H3 pine is typically treated by a LOSP process and the service life predicted in our Technical Design Guide assumes regular maintenance, avoidance of water traps and protection of end-grain.

An economical choice, and a timber that's easy to work with, is preservative treated pine. The two main treatments are CCA and ACQ. There has been some concern about using CCA-treated pine for vegetable gardens on account of its arsenic content, and presumably that would also apply to constructing compost bays. However CSIRO considers it is safe with some minor precautions - refer their data sheet titled The facts about CCA-treated timber, available on the net. If preferred, pine treated with arsenic-free ACQ could be used.

The glue joints should not have opened up. Prolonged exposure to direct sun will lower the moisture content of timber, but you report that the treads in shadow are also separating. It sounds as if the timber had a moisture content that was higher than recommended at the time of installation and is now adjusting to surrounding conditions by drying and shrinking a little. Was any specific moisture content mentioned at the time of supply? Even now it would be helpful if the joiner or the timber supplier could test the timber with a moisture meter.