Did you know there is absolutely NO mention of Imam-Mahdi in the Qur'an? :O Or perhaps alot of Muslims and Baha'is seem to miss that?
It seems weak on how Baha'is will claim that all religions' rituals have been corrupted over time... and yet to establish Baha'u'llah's station and supremacy, utilise those same religious traditions' spiritual narratives as proof of evidential and prophesied advent.
I personally also can never believe in hadiths... they are difficult to historically determine, and if one were to come to the source of Islamic belief system and injunctions, the one thing unifying Muslims of all sects, schools, and stripes, is the Noble Qur'an itself.
The comming of the Promised Ones of God, has always been a Mystery of God.
Please note that, Jesus name was not Mentioned specifically in the Torah prophecies regarding the Messiah. But certain signs instead was given, for people to recognize Him.
Regarding Muhammad also, certain signs are given in the New Testament, to recognize Him. Muhammad Himself said, Jesus knew about Muhammad and told his people about Him.
Also, same goes with the Prophecies regarding the Mahdi. Muhammad told people about Him. But He is not mentioned in a clear way in the Quran, but certain signs are given.
This has been always the way of God, He gives signs, but leave it to Mankind to use their common sense to discover things.
There are many reasons, that the Messenger's name and coming of a new religion is not mentioned in the prophecies of the Books.
Firstly, if the name is mentioned, and comming of a new religion is mentioned, then many people can simply fit themsleves by their name, and decieve many. So, that's why, it is kept as a mystery, and a secrete.
Secondly, the revulations are also a test for people who truely make an effort to find the truth. If God makes it easy, then everyone believes, then how people are testes and how people make an effort?
Regrding Hadithes, I agree, we cannot fully rely on them. But please note that, there are just too many Hadithes about Him, narated from different sects of Muslems, so, It is just clear that Muhammad Prophecized Him.
Regarding the proof of Baha'i Faith, even, just lets say for the sake of argument, that in previous Books, His coming was not mentioned, His proof is Baha'u'llah Himself, and His knowledge. Please note that, this requires an independent, and unbiased investigation of truth, while, a person asks God and put's his trust in God.