The Hadawi Fiqh is without a doubt based on the Ijtihad of Imam al-Hadi and his grandfather Imam Qasim al-Rassi. Ijtihad has always been encouraged, and this is something that the Ahl al-Bayt and Muhaddithin had in common historically. You will be hard pressed to find ´ulama of the Ahl al-Bayt before the 4th century who followed any of the Four Madhhabs, nor will you find them blindly following the opinion of Imam Zayd or others. Their opinions often go against Musnad Zayd, which to some may be an indication that they did not rely on it, and that it therefore isn't authentic.