Yes, I know about so-called incarnations of Vishnu before Krishna. I don't know if Baha'u'llah mentions even Krishna anywhere in His Writings, but he doesn't in what is translated to English. Rama is the one immediately before Krishna, that's all I remember from my knowledge of Hindu. 'Abdu'l-Baha is the first one translated to English who mentions Krishna, but He says nothing about Rama. It's up to scholars and other individuals to decide if Rama represents some earlier Manifestation at the present time. I have no opinion on this. I don't care if he existed or not.
As to Adam, I can only hypothesize that Adam is dimly remembered from thousands of years ago and He was transformed from the first Manifestation of an age to the first person of mankind, and a mythological story was attached to him. His name did not even have to be Adam, in my opinion. The name by Muhammad and Baha'u'llah was conveniently used as was used in the Bible. Someone like Noah was in a similar situation.
Sometimes Abdu'l-Baha talks of Adam as the first man, but that is just a convenience for the listener. He is simultaneously a symbol for the first human and the first Manifestation.