I’ve been having the same problems. I’m pretty sure it’s a FCP bug. I have found a solution though.
go to System Preferences / international / Input Menu : going down the list ‘British’ is grayed out, but this doesn’t seem to matter. go down the list and select US Extended AS WELL. then tick ‘show input menu in menu bar’. At the top menu bar there should now be the stars and stripes, if it’s still British click on it and select US Extended. You will need to do this each time you want to use your keyboard layout.
Go back into FCP and try importing your keys again. It still comes up with the ‘That keyboard layout is for “British”‘ message – but it should switch over anyway.
At this point I resaved my keyboard settings with US at the end to distingush. ( oddly keyboard layout image in FCP is still British even though we are in US settings now).
Save everything and close down FCP. and then reopen. Keys should all be there as they should.
As a test you can go to Keyboard Layout/ Customize – and then flick between Flags in the top menu bar. The keyboard layouts change as you do this.
As a note – I am using a British Macbook Pro with a British Bluetooth Keyboard ( the old white one with numbers on the right). My key layout was created on a Desktop Mac at work – in Britain.
Hope this helps people – i’ve been pulling my hair out for days!
Matt