I don't see any major issue with the decisions of the crew in this instance. Given the chosen cruise level and speed was achieved without the use of MCT. and there are two better suitables enroute, almost certain to have been mid point to the first suitable, please explain the rationale of turning back.
Well according to the ATSB glide range is now a consideration for jet aircraft even though the CAO required 60 mins single engine range for most jet aircraft to operate in is greater than 350NM.