Is there any reason for you to believe that Buddha was a "manifestation" of God?
I don't believe any gods exist. Nor do I revere Buddha as a divine person.
And part of a progression of manifestations that led us to Baha'u'llah? And that Baha'u'llah is the return of Buddha/Maitreya?
No, I consider all humans who have had insights and ideas to be functioning within their own abilities and personalities. No doubt there have been many wise people who never tried to spread their approach with others as a teacher. The few through history who did were surely just the examples that tried and succeeded. Buddhism is at the most basic level a set of mental practices that help manage the emotions and thoughts. It's not magic. Much of Buddhist practice can be said to be consistent with psychological advice.
And... Do your Buddhist beliefs include reincarnation?
It's a fun idea, but Im not convinced. Let's note that how Eastern cultures treat ideas like this is vastly different that how people in the West interpret them. Reincarntion in the East refers to the spirt/moral essence of a person, not like a ghost or spirit that continues on as if still alive.
If so, you know how Baha'is feel about that. And also, What Buddhist Scriptures do you follow?
None. I'm not a person who is attracted to dogma or any sort of guidelines to apply and follow.
Because I've never heard of Baha'is supporting any Buddhist Scripture as being authentic.
It could be that Theravada Buddhism is non-theistic, thus not attractive to Baha'i folks.
Buddhism and Hinduism have had to have major changes made to them to fit into the Baha'i chain of true, revealed religions. And here's Abdul Baha's quote again...
Buddha also established a new religion... He established the Oneness of God, but later the original principles of His doctrines gradually disappeared, and ignorant customs and ceremonials arose and increased until they finally ended in the worship of statues and images.
The quote is inaccurate. First, Siddartha was Hindu, so was born and raised in a polytheistic system, not just one God. And Buddhism was non-theistic as an approach, so inaccurate there, too. So you offer an example of this prophet getting a claim wrong in two ways that has plenty of facts to check. There are religious forms of Buddhism but they are not original.
Plus the symbols in Eastern religions are vasty different than in the West. They tend to represent something real, or a means to a ritual or practice that has some practical end. For example prayer wheels is a reminder of how to be present and aware. The act of spinning prayer wheels is not asking for something from a god or the universe, but about the self and how it has to focus inward, not into an ideological framework.