He is not a fitna, His povs are based on ijtihad.
There was a woman who led prayers in a mosque and she was appointed by the Prophet SAWW. Muhammad Hamidullah in the Emergence of Islam has clearly stated the incident.
And secondly, yes hadith are our solemn belief, but there are hadith that are now being declared weak in nature. I am not saying to oppose hadith altogether, but try to understand his point of view.
His point of views are based on the modern state system or nation state system. If you read his books and articles, you might understand him better, But calling someone a fitnah just because u don't like his ideas is just wrong, This is how sectarianism begins, we don't like the ideas shia hold so should we call them a fitnah aswell?