1) Al-Asbagh ibn Nabata said: I went to the commander of the believers pbuh so I found him drawing lines in the earth, so I said to him: O Commander of the believers, why do I see you contemplating and drawing lines in the earth? Do you desire it? So he said: “By Allah, I have never desired it nor have I ever desired the world, but I was thinking about a man who will be born from the eleventh from my sons, he is the Mahdi who will fill it with justice and equity as it was filled with oppression and injustice. There will be a confusion and an absence for him, people will go astray in it, and some others will be guided in it” So I said? So how long will the confusion and absence be?
So he pbuh said: “six days, or six months, or six years. "
So I said: And would this happen?!
So he a.s. said: “Yes, كما أنه مخلوق and how would you know this matter O Asbagh! They are the best of this nation with the best of this progeny!
So I said: Then what happens after this?
So he a.s. said: Then Allah does what He wills. For verily, He has Bidaa’s and wills, and goals, and ends” – Ghaybat AlToosi p 165 Hadith 127
Bisharat alIslam p60, Kamalul Din waTamamul N’ema page 346
Imam al-Baqir (PBUH): “The companion of this matter will go into occultation in some of these passes” and he pointed out with his hand to the direction of Thee-Towa. Then he said: “Until before his rising, the Mawla who was with him will come forth until he meets some of his companions and asks: ‘How many of you are there?’ They will reply: ‘We are forty people.’ Then he will ask: ‘What will you do if you see your Companion (Imam Mahdi a.s)’ They will reply: ‘By Allah, if he goes from here and takes refuge at any mountain, we will also be with him.’ Then he will come to them and say: ‘Point to ten of your leaders or the best of you,’ then they point to them for him, so he takes them and brings them to their Companion. He will promise them about the coming night..” Ghaibat Al-Noa'mani, P. 187, Tafsir Al-Ayashy, V. 2, P. 56, Bihar Al-Anwar, V. 52, P. 341, Mu'jam Ahadith Al-Imam Al-Mahdi, V. 5, P. 2.
2) Imam Ahmed Al Hassan (a.s) is not a biological son of Imam Mahdi, rather he is his soul son and his real name is Ahmed ibn Mohammed ibn Al Hassan Al Askari. How is this possible?
From Al Sadiq PBUH : If the riser rises, he would have judged by three that none before him judged by: he would kill the adulterer Sheikh, he would kill the objector of charity, and the brother inherits his brother who was in the shadows- Al Khisal, Al Sadooq P. 169.
Imam Al-Sadiq (a.s.) said: “Verily, Allah chose between the souls in the Shadows, then He made the souls enter the bodies. So when our Qa’im emerges, the brother inherits the brother whom Allah chose in the shadows, and he does not inherit the brother from the physical birth. Know the Qaim from that, and whoever does, there remains no further proof!”- Al Haft Al Sharif
So therefore according to Imam Jafar Assadiq (a.s), know the Qaim by him introducing the Soul World, and there remains no further proof that Ahmed Al Hassan is the 1st Mahdi of the 12 Mahdis and the Soul son of Imam Mahdi.
And they ask you, about the soul. Say, "The soul is of the affair of my Lord. And mankind have not been given of knowledge except a little." Holy Quran
3) As for the hadith you mentioned about anyone seeing Imam Mahdi before his rise, I leave you with hadiths that prove that there are people who see him before his Rise.
Imam Al-Sadiq a.s. said: "The Qa'im will not rise until 12 men say that they have seen him, so the people will belie them" - Ghaybat Al-Noamani p277 (in another version page 285)
In ithbat Al-Huda V3 P738 Hadith112: From No'amnai with its Sanad, Imam AlSadiq a.s. said " The Qa'im will not rise until 12 men say that they have seen him, so the people will belie them". Also in Bihar Al-Anwar V.52 P244 Hadith 121 "From No'mani it is mentioned that: The Qa'im will not rise until 12 men say that they have seen him, so the people will belie them". Bisharat Al-Islam Page 22 "From No'mani it is mentioned that: The Qa'im will not rise until 12 men say that they have seen him, so the people will belie them".