In general I would expect the hob cutout to be a single piece. After all the sink cutout is a single piece, if your salesman sticks to his story that it could break if done in a single piece you could ask for the top to be replaced because if someone leans on the sink section it will break! In other words he can't have it both ways!
Obviously it wouldn't break, it should be supported correctly underneath, not a free span.
I have heard of people getting three worktops because of damage to the first two. I make kitchens but would never handle a fragile granite worktop with cutouts, too vunerable. Instead I would pay the high price (maybe 700 a day) to have it professionally installed, then it will be right, if it isn't they will redo it at no cost to me. In other words I am paying for a proper installed worktop and that is what I get.
However if it's not written in stone that it should be a single piece it could be difficult for you. Maybe ring many granite companies and feign interest in a worktop, ask if the hob cutout will be a single piece, if they all say yes then you are onto a winner.
Obviously the kitchen company won't be keen on replacing a 3,000 worktop for free and they will resist it. Maybe just keep the worktop but make so much noise they have to drop the price by 50%, they would probably still prefer to do this than buy a new one. Check out the kitchen showroom, are the tops there in a single piece? If so then you can expect the same.
Cheers
Joe