Link actually made a thread on it near the beginning of the year,which you did reply in but would be good to revisit.
The Mahdi in the Quran.
Also:
List of Quranic Verses about Imam al-Mahdi (a)
https://www.shiachat.com/forum/topic/234991579-verses-of-the-quran-related-to-imam-mahdi-ajtf/
In our Shia tradition we have entire chapters in certain collections such as Bihar Anwar, Kitab al-Ghayba and An-Najmuth Saaqib are proofs derived from the Qur'an, plus there are many scholarly works on the subject.
Same is true for Sunnis.
The Qur'an in it's primary focuses are on the macrocosmic things, not the microcosmic (corporeal) when it comes to eschatology. It's about the planet, the universe etc and it's end.
As for are there ahadith that mention why the Mahdi isn't mentioned explicitly in the Qur'an? well I have come across them before but would have to look around for them.
No, not for arbitrary reasons.
I reject it on the basis of being very late addition historically, as in chronology (via the Shia collectors Saduq, Mufid and Morteza in particular - you won't find the Jesus eschatology prior to these guys, again it's late addition and not the teachings of original Shi'ism), it came from Sunni-Sufis who themselves got it from Christians (Apocalypse of St John et al).
It's not a justified position in Islam IMHO and it is rather demeaning towards Jesus as having not fulfilled his role as Messiah.
I believe the whole idea of the return of Jesus, whilst demeaning Jesus, also demeans and reduces the Mahdi to irrelevance (when he is literally the final messiah of all of mankind, from the Ishmaelite-Muhammadun progeny).
The Judean-Davidic line ended with Jesus. Jesus was the end of Prophecy to the Jews, Muhammad was the one and only universal Gentile Ishmaelite Prophet of whom from his progeny (12 Imams) is the Messiah of mankind. It's incredibly beautiful and consistent.
The role of Jesus exists in a smaller scope and one in the past (one related to the covenant given to the Jews et al), in comparison to the Mahdi who is the Messiah of the rest of mankind.
Anyway, I know as you're a Baha'i you disagree with all of this, but that's not my concern.
Yes, the Shia collection of Hadithes shows the verses of Quran related to the Qaim or the Mahdi.
However, those verses, only according to the interpretations of the Shia Imams are about the Mahdi. None of the verses of Quran can be said is describing Mahdi explicitly!!! No verse of the Quran explicitly says there shall be a Mahdi at the end time.
So, I agree, that the Shia Imams provided an answer here for my OP. The answer The Imams gave, is, the Quran has a an apparent form (dhahir ظاهر) and a deep and inner form (Batin باطن).
The Mahdi is described in the inner form of the Quran.
According to Sadiq, the apparent form of Quran is for ordinary people, but the inner meaning is hidden from the eyes of ordinary men. Had not the Imams revealed them, you and I would not recognize the verses about the Qaim.
But again, this answer is only, based on belief in Shia Imams, and assuming those hadithes of Shia Imams are indeed authentic.
Having said that, the question is now, why the Mahdi explicitly is not mentioned in the Quran? Why is He in an inner form of Quran.
Note that, if the Mahdi was described explicitly in the Quran, our friend
@firedragon would have agreed that the Mahdi is in the Quran. The fact that he does not agree, should tell you, the Mahdi is not seen in the Quran by people. Only Shia Imams interpreted verses of Quran to mean, the advent of the Mahdi.
As regards to return of Christ, there are many hadithes narrated by the companions of the Prophet. According to Wikipedia there are 29 hadithes saying Chrsit prays behind Mahdi.
As regards to Hadithes about the Mahdi, many of these are attributed and chained back to companions of the Prophet, including Ali.
Here is an example of a Hadith which has been narrated on the authority of Jabir Ibn Abdillah al-Ansari that I heard the Messenger of Allah saying: "A group of my Ummah will fight for the truth until near the day of judgment when Jesus, the son of Marry, will descend, and the leader of them will ask him to lead the prayer, but Jesus declines, saying: "No, Verily, among you Allah has made leaders for others and He has bestowed his bounty upon them."
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Hadith of Jesus praying behind Mahdi - Wikipedia
So, no, it is not a later addition in the Sunni collection. If you were to say these hadithes are false just by saying they are in the sunni collection or by accusing some companions of prophets as liars, this is just same old fight between sunni and shias. Sunnis also say, the entire Biharulanwar is fake, because it was written more than 1000 years afterward.
So, let's look at these unbiased please.