The whole argument probably depends on being able to recycle the CO2 by growing more corn to turn back into ethanol, instead of going to outlandish places prospecting for more fossil fuel.

The amusing thing is that we are also told there is a 'shortage of CO2' for making dry ice as a refrigerant for transport .... :-)

As has been said, the emitted CO2 is balanced that that absorbed when the maize or whatever was grown. Contrast with petrol where the CO2 released was locked up millions of years ago, so is "new" CO2.

But to be CO2 neutral, all the energy used in produced the ethanol (ploughing, planting, fertilising, harvesting, distillation, transport) would have to be renewable. It seems unlikely that will be the case. You would also have to consider the effect on food production of taking the land out of food production. If growing food becomes more intensive then that could also be accompanied by negative impacts.

Somebody must have done the sums on this but it sounds as if it could be a fairly small benefit especially at the level of adding an extra 5% ethanol to fuel.

Good question.

I'd like to see the answer in full if it exists, and also for that matter the potential benefit of heat pumps. Heating a home using an electric heat pump should use only about a quarter of the energy that heating it directly with resistive heating does. You can use renewable electricity for both of course but as with growing cops for ethanol there is a lot of hidden
carbon being added to the atmosphere somewhere in the processes of making wind turbines, pv panels, tidal barrages or whatever. The combination of proper insulation and heat pumps for home heating must be fairly substantial.