With the deck being quite narrow have you considered taking the timber to the edge of the deck and not using a coping stone. This will give you a sleeker more clean lined look. With regard 90 verses 140 decking. We use mainly 140mm now, but with the deck only being 2 metres wide this may look out of place. A lot will depend on your surroundings. If the deck hasn't been started yet I would also suggest that you run your decking the short length, this will draw your eye along the lines and give an illusion of the deck being wider than it is and looking up it not as long. With regard cupping it very much depends on the timber selected and you really need to screw down rather than nail. For almost all of our decking I specify and recommend Accoya for its stability. I am sure that it will be available in Australia.

extra fascia depth would be dependent on how much you want to spend on extra labour by not using standard steel fascia (which is only about 185mm wide) with all the standard fixings ..A carpenter or plumber should be able to make a wide substitute out of sheets of ribbed colorbond steel and the rest should be standard timber framing and soffit linings but check wit a carpenter or roofer about what you want to achieve (why does it need to be 40cm wide??) and there may be a very simple alternative available

Depending on your budget and how much structural work you want to have done, you could keep most of it the same. But you would loose your meals room. extend your laundry to the door of the meals room, turn your laundry side ways this will give you access to the out side, on the other side of the laundry extend your wall pass the internal door of the meals room to create a walk in pantry, extend kitchen gallery to laundry and pantry, open up internal walls of kitchen to living room and dinning room creating a beautiful Island bench looking into living room. open up external dining room wall for sliding doors to bring the inside out. because there is a walk in pantry there would be no need for upper counters so u could also have a bar at the kitchen window on the outside. Doing it this way you would save a lot of money not having to move plumbing and electrical in the kitchen. You would have to walk through your kitchen to get to laundry though.

your designer should be able to specify a suitable size depending on teh material you plan to use and orientation...what are you trying to screen?