Hadith is not words written in stone. I am not sure i am may be wrong but from my knowledge there is no mention of Dajjal or Imam Mehdi in Quran.
Only Quran is protected as word of God not Hadiths which were twisted to the needs of Mullahs over period of time.
Lastly this conversation is not even relevant
Quran is Ummul Kitaab. If Quran would have contained everything in detail, Quran would have been manifolds larger than what it is. Quran is tha manuscript and Prophet (SAAWW) is the key
It is He who has sent among the unlettered a Messenger from themselves reciting to them His verses and purifying them and teaching them the Book and wisdom - although they were before in clear error -
62:2
For example, Quran only tells us about Praying, not how to pray.
Quran asks us to Fast but not how to Fast.
So the Key to Quran is the Prophet's acts and sayings
And so if you don't accept the authenticity of Hadith (Which is a multi-dimension subject in itself) your acts of faith (Salah,Fasting,Hajj etc) themselves become dubious as you doubt the origin.
For example, the Quran says
6-158
Do they [then] wait for anything except that the angels should come to them or your Lord should come or that there come some of the signs of your Lord?
The Day that some of the signs of your Lord will come no soul will benefit from its faith as long as it had not believed before or had earned through its faith some good. Say, "Wait. Indeed, we [also] are waiting."
Here is an abstract from Tafseer Ibn-e-Abbas (RA)
Now this sign is detailed in Hadiths. Not in Quran. However, some commentators argue that the use of plural in this ayat
means this ayat implies multiple signs . For example, Imam Ibn-E-Kaseer (Rehmatullah E Alahe) comments on this ayat as following
So Dajjal is included in these "Signs" along with Sun rising from West and Da'aba tul Arz.
Regarding Imam Mehdi
And who doth greater wrong than he who forbiddeth the approach to the sanctuaries of Allah lest His name should be mentioned therein, and striveth for their ruin. As for such,
it was never meant that they should enter them except in fear.
Theirs in the world is ignominy and theirs in the Hereafter is an awful doom.
Imam Ibn e Kaseer (RA) is of the view that this also implies the time of Imam Mahdi (RA)
So not combining Quran with Sunnah (which is itself transmitted through Hadith) leads to doubts and hence undermines the whole belief.
Sorry that was off topic but i hope it will help.