When people say that quartz looks like "fake" stone it always makes me scratch my head. For me, I love it because it is clean and uniform--if I wanted "real" stone with the variability that comes with real stone, I'd choose granite or marble. Saying it looks like fake stone is like saying that cork looks like fake wood. Two very different materials, two very different looks. Neither one is better--it's what style you prefer. Exactly! But I have found that many people try to sell quartz counters saying they look like granite but are better/easier/tougher (which I think has already been addressed above). Some of their colours do appear to be in fact imitating the look of real stone. I don't think they do well at that, but I totally agree with you that it's a great choice if you want a uniform or clean look or their vibrant colours.

You will always see the seam. Very rarely do they completely disappear. A good fabricator will make sure the seam matches in pattern. Are they wanting to use two slabs with the seam running through the 74" depth or do they want to run the seam down the entire 98" run? Either way, that's a large seam. I would recommend either reducing the size of the island to 65" (or whatever the size of your chosen slab is) or select a natural stone that is available in a piece large enough to do the island without a seam.

i wouldn't be too worried about seam strength, frankly. if it's in the corner, the cabinet edges will help support it, and similar would be true for the sink. i would think about what you would find noticable/irritating. myself, i'd want the seam in the corner. i'd find it less noticable there.

Use the 6". I wouldn't worry about this.