Do you want to keep the "black falcon oven" as the feature or do you want the island to be the feature? If you want to keep the oven as the "feature" in the kitchen, then I would do the island in the same cupboards and benches as the rest of the kitchen. If you wanted the island to be a complimentary "feature" to the oven, then you could perhaps do the same cupboards, but go with a black benchtop (although I would personally avoid a black benchtop due to it easily showing dust, water spots, etc). What material will be "under-bench" be on the island bench? Is it also going to be a feature (e.g. a graphic print, timber or something else)? Will the island bench have waterfall ends? If the island bench already has some level of visual interest, then it may not need anything more (i.e. different materials). You can always add visual interest to the island bench with accessories.

Your view may be the wow factor- You may be better to put your wow into the splash back as ultimately it is less expensive to change. A total rebuild has many areas for cost over runs so with a large area, I would look at your overall budget and how much you are allocating for bench tops - This will help determine what you can afford and ultimately your choices. Depending on the surface dark benches can be harder to keep looking clean. I suggest you borrow some samples and bring them home - wet them and wipe them etc

I just googled & found this site, which quotes cost of R2.5 75mm. there are other suppliers as well. What was quoted to you for cost of roof installation, and was it itemised so you can see exactly what thickness the foil-backed insulation builder will use? I'm guessing the thinner (lower R factor). So, the new quote should reflect a credit for the cost of previous insulation, since it won't be used, then cost of thicker insulation added. Does that make sense? If you are unsure, best to speak with the builder. https://www.pricewiseinsulation.com.au/product/heavy-duty-fletcher-permastop/

I wonder how much time you might have to spend cleaning the roof behind the parapet, perhaps the rain will take care of that, leaves?. If you were going to use these spaces as terrace you would need to adjust the height of the parapet or add a railing, if not the height of the parapet could be reduced