That is what I tended to assume, that lithoautotrophs were first so to speak. But that is not the prevailing hypothesis. The prevailing hypothesis is that organic molecules were present in considerable abundance, massive amounts. Given this soup rich in organic molecules derived from the basic chemistry of what was there then: heat, ammonia, methane, sulfur, lightning, water, solid catalysts (clays such as kaolite etc can drive nearly solid state chemistry) ... a rich soup was formed. The food for heterotrophic (chemoorganotrophic if you will) life was already there. No reason to go with the lithotrophic model, the energentics of which are considerably less favorable.
So one has a soup of organic molecules, they stumble into one another for a billion years, and some of the stumbling makes more of self. Substrate is not limiting in this model. One need not evoke anything particular tricky, any single event that makes more of itself will become dominant. Take any bucket of things, if there are things in there that make mroe of themselves, they will overpopulate compared to the things that do not.
On the hurdle to life bit, it seems likely from the prevailing hypothesis that first life derived energy from eating organic food that was available in abundance.
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