Hi mate, I've done a couple of trips so only have limited knowledge but have learnt a few things.
1 - Plenty of GT's about, Western side reefs hold nice ones and diving at Tryal rocks I saw one about 40kg
2 - Be wary of launching at lower tides, the rocks are very slippery and the ramp is steep, I've pulled 2 cars out that have skidded in the water when launching the boat and not stopped when the brakes were put on.
good idea to launch from there as it cuts down the ocean miles you need to travel.
3 - Yes, you have to check the depth of the bays so there is water there at low tide (depending on your boat and if you want it dry or not) There are plenty of beautiful bays to camp on
4 - Never 100% but generally May and October are good months
5 - Do the maths on your fuel usage and how far you are looking at travelling. Add at least 30% and you should be right. I always check the amount of fuel used on the trip over and allow double coming home, the rest is playing fuel. If you get caught in some rough water on the open ocean back to Fortescue you will use heaps of fuel and you are a long way from any rescue boats! 350L seems a bit light on for 5 days though. Don't know what motor and boat you have so hard to tell.
research on this forum mate as there is plenty of posts and info on it. Good luck with the trip and make sure you post up a report :)

 I'm looking at the same thing mate be goin up in October for a week so keen to hear people's hints and tips

 heaps and heaps of info on this site

you can only camp on some islands, do you research on dpaw website

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