Making the zebra more like the horse, or making hippos like Indian elephants, does not seem to require massive surgery in evolution.

and the abundant resources available to civilization would accelerate subsequent technological development as well.

That's a valid point, and could be analysed further.

Many of your proposed counterfactuals seem to have the same flaw, implying exponential growth with slightly different coefficients rather than a completely broken model.

Different coefficients and starting times is enough to break Robin's model. And for Kurzweil's model, I can wipe out the whole human species at critical moments, and still be inside it.