Could we use a free API key for release on our corporate client's projects or we must get a paid key when it goes for release. Is there restrictions or exceptions over the free API key (other than the query limit).
You can use the free Developer level key until you go over the 500 per day limit. Then you should upgrade to the right paid tier. The logo needs to be displayed where the weather is, even when the API key is being paid for. When you upgrade the key it stays the same though, so you won't have to change your code.