Hinduism religion does not believe in a single founder of Hinduism. The reason is apparent. Following spirituality as detailed in scriptures of Hinduism seekers reached stage of enlightenment, self realization, god realization and finally state of moksha salvation.
Emancipation from cycle of birth and death is the only goal of life for a seeker in Hinduism. In the circumstances, searching for a single founder becomes meaningless.
When human intellect, mind power reached a particular level, highly evolved Sages and Rishis invoked jnana wisdom directly from God (Brahman in Hinduism). These initial words of God were called Vedas which are four in number.
Subsequent to Vedas many sages and Rishis indulged in independent treatises to explain the philosophy contained in Vedas. As Vedas were voluminous documents beyond reach of masses even scholars, the deliberations by Sages and Rishis made it easier to understand them.
But Upanishads deal with only spiritual aspect of Hinduism, not religious. It is only spirituality indulging in which human beings finally reached God, never religion. All Sages and Rishis were primarily interested in spiritual wisdom.
As jnana wisdom contained in Vedas (Hinduism revelations) were still beyond catch of masses, advent Lord Krishna and along with him the wisdom of Bhagavad Gita. The teachings of Bhagavad Gita were not only easy to understand by masses but also scholars. Finally wisdom contained in Vedas, Upanishads reached masses galore.
In present times all Hindus viewed teachings of Lord Krishna in Bhagavad Gita with utmost reverence. There was nothing more pure than teachings of spirituality contained in Bhagavad Gita. One single scripture Bhagavad Gita suffices in present times to reach state of enlightenment. The only requirement was becoming a Swami Vivekananda in real life.
Contents of Bhagavad Gita cannot be understood by all, not in the least literally. All scriptures including Bhagavad Gita demand human beings mastering contemplation (chintan) to reach the hidden underlying meaning. In absence of requisite contemplation (chintan) spiritual progress was dismally low. There also was no shortcut for assimilating jnana wisdom contained in Bhagavad Gita. For understanding Bhagavad Gita knowledge of Hindi and Sanskrit was not necessary but preferable.
Authentic commentaries on Bhagavad Gita in English are available with Gita press, Gorakhpur, India.
Hinduism primarily is a collective thought process that was finally scripted into written form.
For Hindus it was immaterial who founded Hinduism, what mattered was our indulgence in spirituality to reach cosmic end of life, state of self realization, god realization.