You're right that the seats are not the car's fault, but it's the car manufacturer's fault when the only seats available are not suited to a variety of body types.
Really? How many manufacturers you can name that offer "comfort" seats in their sporty models? Si, Type-R, WRX/STI, ST/RS? Any other? I think they all offer "sport" seats in their sporty trims/models and that's appropriate. Sure more or less bolstering, but definitely significantly more aggressive than their regular seats. VW is no different and it would be a shame to be.
Why should only small people be comfortable in small cars?
I don't know, I think it's physics. A larger person takes up more volume from a given, smaller, volume available in a small car, leaving less free space and resulting in a cramped feeling. The size of the seats is also constraint by smaller volume in small cars.
As for volume controls accessible to a front passenger, Audi and Mazda put the volume control on the console. It's a good design. I wish it were more common.
I think this comes down to personal preference. I prefer all controls on the center stack rather than floor console. This way, you can use your peripheral vision to find them while still looking at the road rather than having to look down and take your eyes off the road. More so, for those frequently bringing drinks into the car is more likely to spill stuff on the switches which are harder to clean. I prefer a clutter free floor console. If anything, I would like the space to be used for small items storage.
Also, having literally just tested this car, I will respectfully disagree with you that the GTI's volume knob is close to the passenger. If you're tall and adjust the passenger seat far enough back to accommodate long legs, it's a long reach to the left side of the centre stack.
Having lived with a GTI for more than 3 years now as my daily driver, I prefer the driver oriented cockpit rather than passenger. Ultimately, everything should be ergonomic primarily for the driver (always in the car) rather than the passenger (occasional). You got me curious and I sat in the passenger seat (setup for me) and I can reach the volume knob with my back pressed against the seat back. If the passenger needs to reach the volume knob they can stretch a bit if need be. It's not like they have to pay attention to the road or anything. I honestly don't see the issue here but we can agree to disagree.
Yes, my wife's arse is larger than mine (she is not reading this forum so I'm safe to say it) and she doesn't particularly like the aggressively bolstered seats in teh GTI either, but who cares? It's not the car's fault (or VW) that she has a bigger arse that doesn't fit well between the bolsters. I for one, hate the flat seats in any car. I like my lateral support.
One other thing that caught my eye in the article.... "
...The other is the artificial engine noise the GTI pumps through the speakers."
While I know there are cars which route simulated engine noise through the speakers, this is not the case with the GTI. The GTI has a physical electronic device ("sound actor") located under the plastic cover at the bottom of the windshield (on the outside) that, when activated by the Mode settings, it takes the engine noise and creates an enhanced simulated/synthetic engine noise inside the cabin. This device can be programmatically disabled/volume adjusted via OBD or physically removed from the car if so desired (relatively easy to do). It can be also disabled or at least toned down using the Mode menu. Certainly, the enhanced engine noise is not played through the speakers.