In theory a lot of what Baha'allah says is true if he would be a Prophet of God. But none of that actually is proof that he is a Messenger of God. Just like what the other fake Mahdi claimers if they claim to be from God, they are right per Quran, we should accept them. But that's a big if.I agree and that is exactly what Baha'u'llah wrote:
“It follows, therefore, that every man hath been, and will continue to be, able of himself to appreciate the Beauty of God, the Glorified. Had he not been endowed with such a capacity, how could he be called to account for his failure? If, in the Day when all the peoples of the earth will be gathered together, any man should, whilst standing in the presence of God, be asked: “Wherefore hast thou disbelieved in My Beauty and turned away from My Self,” and if such a man should reply and say: “Inasmuch as all men have erred, and none hath been found willing to turn his face to the Truth, I, too, following their example, have grievously failed to recognize the Beauty of the Eternal,” such a plea will, assuredly, be rejected. For the faith of no man can be conditioned by any one except himself.”
Gleanings From the Writings of Bahá’u’lláh, p. 143
The Quran shows consistency in the religion of God, and so yes, denying any new Messenger would be wrong. However, Baha'allah interpretation is problematic, contradicts the Quran, and is impossible.
His interpretation that Mohammad (s) being stated by the Quran that he is the Final Nabi is in the same way Ali (a) or Mohammad (s) represents Adam (a) and is metaphorically Adam (a), is a game on language. There are expressions allowed to say you are one, those expressions are found in the Quran and hadiths, but they have their proper usage, terms, angles, context, and flow. It's clear, that when Quran says Mohammad (s) is the final end of Nabis, it's not hyperbola nor is it talking about unity of the final Prophet (other then Mohammad (s)) and Mohammad (s) being one. It's absurd.
But for the sake of helping you guys, I didn't just hold this to account. I entertained say it's possible. Bahai Faith contradicts Quran in other respects even aside from the day of judgment and finalizing of Nabiyeen. It contradicts the viewpoint that there is a living guide/witness/leader in all times in the Quran for example. Baha'allah and Bab not being relatives also violates a major principle of how God weeds out false claims to succession per Quran and that blood relations matter to God for a wise reason.
The twelve number is violated with respect to succession and months as well.