I do not know just how the official buttons are constructed, but speaking with some mechanical knowledge as well as some knowledge in fabrication, I have to say that making a 250mm button that moves in a parallel manner when force is applied to any arbitrary area of that button is going to be a difficult task. Not impossible, but difficult. Some devices achieve this to a limited degree. For example, the space bar on the keyboard I am using attempts this but is only partially successful. I can see a noticeable angle in it's orientation when I press it at either end. So, it works, but is only partially successful at it.
So I would abandon the idea of any requirement that the button must move in a parallel orientation. Somewhat parallel, perhaps, but not completely so.
I would construct the button itself from a type of plastic that can withstand abuse. Nylon, for one, comes to mind. Near the four corners I would place bolts with springs around them. The bolts would enter from the rear through holes in the framework. The four springs would be selected to provide the needed resistance to support the button while allowing it to be activated by the specified force. I would have those springs inside some counterbores in either the button or the supporting frame or both. The bolts would be selected and adjusted to provide only a small amount of movement, perhaps 2 to 5 mm.
Since the button plate is allowed to tilt at an angle, the switch must activate with different amounts of travel. This must be taken into account. One method would be to place multiple switches, wired in parallel, perhaps at the four corners. That would work and it also adds redundancy. But it is more complex and will cost more. And they would still need protection against high forces when there is over-travel so that complexity is needed four times.
At the center of the supporting frame I would place a single, good quality switch which has some allowance for over-travel with no damage. The micro switch with a spring arm is one way. Another would be to use a spring between the switch and the button. Either approach would work. You could even fabricate a spring style, contact but a commercially available switch would be far more reliable. The position of the switch along with the spring would be adjusted to allow positive activation no matter where the button is pressed.
By using rather large screws/bolts at the corners the device should be strong enough for human abuse. I would try 6 mm bolt size at first and move up from there if problems are found. A high grade bolt would help. The springs should have a small movement vs. their overall length and diameter for long life.