It’s funny you ask about Hinduism, CG!
To answer your first question, the Adamic Cycle was not the first cycle of Revelation. There were many before that. The definitive list of Manifestations from this cycle, from my understanding, would include (in no particular order) Adam, Abraham, Moses, Zarathustra, Shakyamuni Buddha, Jesus Christ, and Muhammad.
You’ll notice that I left out Krishna. This segues into my answer to your question as to why Krishna (and not any other avatara) is counted among the Manifestations. To speak to Hinduism in general, we Bahá’ís do not teach that Krishna, the Mahabharata, the Bhagavad Gita, Vaishnavism, or the like is representative of all of Hinduism. We recognize that what’s known as “Hinduism” is actually a vast array of local religious traditions centered around various deities and practices, so we try not to generalize here. Or, really, with any religion. No religion is a monolith.
Krishna in His original context — from what I understand — is actually as being one of a number of avataras that Bahá’ís recognize. He was not the first, nor the only one. There were (to name just two) Rama and Narasimha. The reason as to why Krishna was chosen is, perhaps, because of the wide-reaching influence of His Name (being one of Hinduism’s most popular deities), with the Mahabharata and the Bhagavad Gita as widely familiar texts. This is entirely independent of the Bahá’í Faith.
So, when the Central Figures speak on Krishna or on Hinduism, they speak from a general awareness of it, not from anything really detailed that They knew. Shoghi Effendi encourages Bahá’ís to learn more about Hinduism (and, indeed, all religions) and in their own understandings, account for scholarly developments. Such references in the Writings are not for the purpose of saying that Bahá’ís believe that Krishna is the only avatara in Hinduism, or that Vaishnavism is the only legitimate Hindu sect, or anything of that sort, or that Krishna is somehow the “Founder of Hinduism” (whatever this means). What the inclusion of Krishna specifically is designed to say is that Hinduism in general is recognized by Bahá’ís as having survived and are given supreme legitimacy (not that other religions unmentioned are not legitimate, except for a few). All of these accusations are incorrect.
Coming from all of this, I’ll end off by saying that the criticisms leveled of how various Bahá’ís relate to Hinduism based on only a small handful of references to Hinduism in our Texts is unfair.