Some questions you raise are easily answered, some not so much. At least by me. Since this question has no answers I ll attempt part of it.

With the decline of Sanskrit, we see a rise of apabhramsa ( corrupted ) languages. Out of this, came most modern northern Indian languages, including my own first language Gujarati. These corrupted forms interacted with Prakrit ( natural languages ) and some foreign languages ( Persian in Gujarati s case ) and among themselves. Hindi arose from the sauraseni apabhramsa.from the same, came the dialects commonly spoken in UP and Bihar. Since these languages shared common ancestor, and since the “Hindi belt”today comprising of nearly four hundred million people ( UP, Bihar, Jharkhand, Madhyapradesh ) it’s easy to see that Hindi ( or Hindustani as I later explain ) has always been dominant in India.

The Hindi spoken today in common parlance and Urdu are quite similar, they are in fact two forms of Hindustani. And may be your question is about that rather than Hindi. As someone taught classical Sanskrit, I still struggle with pure Hindi. It is easy to slip into Hindustani for me, since the Persian derived words in my first language Gujarati and Hindustani are the same or similar. This is despite Hindi and Gujarati having the same “tatsam” ( as is ) words for the same things taken from Sanskrit.

Hindi as a language arose in opposition to Urdu as a pure Sanskrit derivative. In that it’s more artificial than more natural dialects found in the region. It’s propped up by the fact that it’s a language taught in most schools in north and west India as a first or second language.