So the question to answer is WHY is it required to address all these contradictions at once?

This is because all of the 3 major contradictions of American society (capitalism, settlerism and imperialism) are already "matured" contradictions. Instead of developing "positively", these modes of surplus extraction are all running into their ultimate limits. In most AES societies, capitalism had developed to a very limited extent, which is why most AES societies had to/will have to undergo quite a bit of development before they could/can abolish commodity production, which is the ultimate source of all of modern society's contradictions.

This is not the case for the US. There is no need to undergo a period of state capitalism or anything similar. There is no "primitive" development to wait for. All of the 3 major contradictions of the US (capitalism, settlerism and imperialism) are in their terminal phase right now. In fact, the material basis for imperialism (which is dollar hegemony) and settlerism (which today in America I hypothesize is suburban development [1]) are already collapsing, and may collapse even before a worker's state is established.

My interlocutor’s position was that the bourgeoisie would never allow the working class to build a movement with the indigenous to such a mature state

They will never allow the working class to build a movement, with or without the indigenous.

Therefore, his position continues, the primary contradiction is between the proletariat and the bourgeoisie, and resolving that contradiction creates the material conditions necessary to solve the other contradictions.

It is the other way around. The American bourgeoise possess basically unlimited purchasing power because of Imperialism, and settlerism has allowed them to buy the loyalty of the American working classes. As long as America remains settler and imperialist, the bourgeoise in America cannot be abolished.

Part of that position is that the indigenous population in the US is simply too small, too divided, and lacks sufficient power over productive forces to have revolutionary potential.

It is the task of the revolutionary movement to change this fact. If revolutionaries cannot even win over and organize the most fucked over victims of American capitalism, how are they supposed to overthrow the government?

But is there more I could read to strengthen my understanding of this topic?

I am honestly not aware of any marxist works that deal with the present state of class contradictions in American society with the level of broad overview necessary to see the whole picture. My position that I am articulating is largely the synthesis of what I have learnt from other comrades and from American history

[1] even if it isn't, my point still stands, as the growing housing and healthcare crisis is proleterianising the latest generations of Americans en masse.