"The key difference between the two results is that B assumes that half of the corporation income tax is shifted to consumers, in the form of higher prices, while A assumes that all of it is borne by shareholders, who are generally high-income taxpayers."
Bogus assumption. Corporations (which don't pay taxes anyway) cannot control how much of the tax they pass on to consumers in the long run or even year over year. Market forces do that.
That is right, and the assumption is that about half is in practice. It is driven by the elasticity of the cost of capital actually. The higher the elasticity, the more of the corporate tax will be passed on in the form of higher prices pursuant to market mechanisms.