So, if I bought one of these from USA car, would it fit?

If the centre console is the same size and dimensions on the US cars, and the fitting lugs are in the same place, it should fit. I'd expect them to be the same, but ... you can't just guess, or hope.

Now, do you know anyone who regularly takes these cars apart, and might be able to measure a UK one? You?

Ask the seller to give dimensions, and to turn it over so you can see if it looks similar. Also need to measure the gearlever hole, as that might be different. Check the window switch connectors. Again, I'd hope Fiat were not daft enough to create a different connection, but best to check. If all checks out, might be worth a punt.

No they do not fit. You'd essentially have to swap over the whole console and dashboard if you want to use OEM parts.

I'd like to eventually fit the round USA/2016 switches to my car, but the only way to go about it would be to adapt them to the older EU console (cut and paste), then have the whole thing covered in leather or carbon fibre or some other finish.

The round switches have been added to the latest EU cars too, but again, with the US style console and dash.

Oh bum! I was really hoping they would fit.