1. The Yamani is a sign of Allah. A proof of Him, a Hujjat of Him, as described in Ahadith. Bear in mind the Yamani is not Imam Mahdi (a.s). The Blessed Yamani is the Messenger of Imam Mahdi (a.s). This man is to come at the end times. Read this Hadith:
"Hijaz (Saudi Arabia) will be ruled by a man whose name is the name of an animal, when you see him from a distance, you would think he has a lazy eye, and if you get close to him, you do not see anything (wrong) in his eyes. He will be succeeded by a brother of his, named Abdullah. Woe to our Shia (followers) from him! Woe to our Shia (followers) from him! Woe to our Shia (followers) from him! - he repeated it three times - Give me the good news of his death, and I shall give you the good news of the appearance of the hujjah (Imam Mahdi)."
Reference: Two Hundred and Fifty Signs, sign number 122
It is the lifetime of King Abdullah of Saudi Arabia. He will die soon one day. When he dies the Imam (a.j.t.f) will come. But before he (a.s) is destined to come, the Yamani is to come, as the appearance of the Yamani is a sign of the Imam's (a.j.t.f) coming. Thus, the Yamani should come during/before King Abdullah of Saudi Arabia's lifetime.
Ahmad Al Hasan is the only man with clear proofs who has claimed to be the Yamani.