Without wishing to get into any dispute, or indeed to be accused of hindsight, I do think 15 minutes extra in TS conditions is a bit tight. Especially when all the alternates (indeed the whole region) seemed to be having the same weather. I guess you could say the skipper was carrying enough to hold and allow a single cell to clear the field...15 minutes might do it...but all the same I personally would like more. Maybe they had weight/range probs? I'm not familiar with the type.
As to the crews competence, I may be reading it wrong but I thought the F/Os input sounded OK. He seemed to make all the required callouts (e.g spotting the 1000fpm descent and calling it). Given his low time I expect there was (even with the best will in the world) quite a steep cockpit gradient.
Incidentally, do Aussie rules require an extra 1hr holding fuel when CBs are forecast? Heard so somewhere.