I dont think you can find any western manufacturer to agree to 6 month delivery. Maybe you can try with eastern (India, China) manufacturers such as SPECO or CHINT.
Technical brochures (TB) of CIGRE may be of help. TB 358 is about "Remaining Life Management of existing AC Underground Lines", while TB 761 asses power transformers. For transformers specifically, this thesis covers the topic, and can guide you on the specific standard for the remaining life...
In my experience is not common for the engineering company to provide the simulation packages or the study. Since the study needs to be reviewed/updated every 5y, or every time the topology changes, it's not in the best interest of these companies to give you the .oti files.
Seems like ETAP is not considerign the section above because you didnt define a duct in that section. Have you tired using a direct buried raceway instead? Since both RHOs of your concrete and backfill is the same, im guessing you are better off using a raceway with a 40 C.cm/W of 25x60cm.
As...
