“Je široká ulice pod našimi okny” is not accepted (I would have guessed these were interchangeable). Is this order grammatically wrong / unnatural, or does it alter the meaning?
It's extremely unnatural.
The best are 'Pod našimi okny je ŠIROKÁ ULICE.' or 'Široká ulice je POD NAŠIMI OKNY.'
The capitals indicate the emphasised part.
Thanks for the quick feedback :)