I have to break it up. Since the questions weren't question/answer, this will be all over the place.
That's definitely a good belief. Many religions do share this as well. All is good.
Does Baha'i religion replace older religions (in order to achieve this goal)?
The oneness of mankind would involve replacing other religions in order for them to evolve into one that promotes oneness (so, disregarding teachings of that time period that teaches the opposite)?
Shaping religions in order to promote oneness bahai religion defines?
We agree with the oneness of humanity. We disagree with the religious part, though, not the humanity part.
That's good for many religions. I know other religions that do this too; that, not the focus, though. We're talking about the religion part of bahai (god, manifestations, etc) rather than charity and humanity.
Does Baha'i believe that Hindus, Christians, Muslims, Buddhist, believe in the same abrahamic god?
Yes. The issue isn't the oneness of humanity. It's the religious part.
God: Are all religions under one abrahamic god?
Manifestations:
a. Do manifestations replace "old" teachings in place of new ones (that relate to oneness)?
b. Are other religions (Hindu, Buddhist, Muslim, Christian) wrong because they hold on to old beliefs that do not help humanity in today's period?
Years ago, one Bahai mentioned that scriptures were distorted. I can't quote verbatim.
Scriptures: Do bahai change the scriptures (Suttas, Bible, Quran) to its original in order to promote oneness of humanity rather than division?
1. As to God. This is how Baha’u’llah puts it.
There can be no doubt whatever that the peoples of the world, of whatever race or religion, derive their inspiration from one heavenly Source, and are the subjects of one God.
So He is saying no matter how we term it or understand it, it’s the same heavenly Source all religions derive their inspiration from.
a) Jesus said to love thy neighbour, with Muhammad He said to love ones country and with Baha’u’llah He said to love all humanity. So as the world progressed and expanded so too the Manifestations expanded the concept of oneness and love which incorporates the previous concept for love of ones country includes love of ones neighbour and to love humanity includes nations, individuals as well as races and religions.
B) All the religions you mentioned are true. We read from all their Holy Books each week in our Houses of Worship all over the world. (Quran, Bible, Bhagavad-Gita, Zend Avesta etc)
Actually the more each religion follows and obeys its own teachings the better humanity will be. So if all religionists , including Baha’i, strive to live according to what each religion teaches us we would have had world peace long ago.
c) The scriptures of other religions we do not believe are distorted but interpretations can be distorted. The further back in history we go the more difficult it becomes to authenticate the true author so many religious texts remain ‘attributed to’.
In the Book of Certitude Baha’u’llah points out that with so many copies having been distributed of Holy Texts actual physical distortion is well nigh impossible without being detected but by corruption of a text He said is meant ‘corruption of the meaning’. So some have twisted the meanings of Holy texts to provoke war which was never the real intention.
D) No Baha’is do not change any religious text. But we read them as devoted believers would because we believe in them too. So I read the Dhamapadda as a Buddhist would as I believe in Buddha and the Gospels I read with devotion as a Christian would with humility and utmost reverence.
This is an inscription Abdul-Baha, Son of Baha’u’llah, made in a Bible in London.
Inscription in the Old Bible Written by ‘Abdu’l-Bahá in Persian
THIS book is the Holy Book of God, of celestial Inspiration. It is the Bible of Salvation, the Noble Gospel. It is the mystery of the Kingdom and its light. It is the Divine Bounty, the sign of the guidance of God.
‘Abdu’l-Bahá Abbás.
With such reverence towards all other Holy Books to Baha’is these are all part of our own belief and so we don’t change them.