The roof itself is think sheet steel, yes, but the structural steel running from the A to C pillars is enormously strong.I feel like it's worth pointing out this section of the manual, which was somewhat glossed over in that particular thread. I'm 99% certain your insurance would be invalidated if you attached anything to the roof, beside the fact that it could well damage the car. I did notice the other day when it was raining that the roof does sound very thin and tinny in comparison to other cars I've had. Not a complaint, just an observation, it doesn't matter for normal use of course.
As for legality, we’re back to the old chestnut about whether it’s legal to fit a tow bar to a car not type approved for it. The answer to that is it is legal for carrying bikes, and I don’t see why a properly designed and fitted roof rack system wouldn’t be in the same camp.
But yes, tell your insurer as well.