I don't understand, if the original essence of Hinduism is monotheism and acknowledgement of Brahma as only God (Ishwar), why do Hindus need to incorporate what we would consider polytheistic beliefs and observances? See below from the Vedas:
We may postpone this.
Yajurveda 32.11
Ishwar resides at each point in universe. No space is devoid of Him. He is self-sustaining and does not need help of any agent, angel, prophet or incarnation to perform His duties. The soul which is able to realize this One and only One Ishwar achieves Him and enjoys unconditional ultimate bliss or Moksha.
[Ishwar means God in Hindi]
Actually IMO first we must invert the understanding. It is not Brahman or Ishwara that resides at each point in the universe. Rather Space is in Brahman. All the universes of waking, dream, and sleep states that we traverse daily subsist on Brahman, which of course is transcendental to all our universes and also immanent in all. The very act of seeing and knowing happen because of it. And sans our forms we are brahman. So, it is said Ayam atma brahma (this Self is brahman). Easiest approximate analogy is of water, sea, and waves. Water is the essence (Brahman), ocean is Ishwara, and waves are all forms. The logic is that just as all forms of gold are actually gold only, all forms are Brahman alone.
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This is my understanding based on Advaita vedanta school of Hinduism.