"Well now, if I were to process the crops into biofuel using energy derived from that (or similar) crops, which were also processed using energy derived from that or similar crops (and hence recursively)... biofuels would not emit any net carbon."
When you begin to factor in all the energy losses in your recursive system, you will need progressively more crops to produce the resultant biofuel energy source, a curve which will, eventually (read "quickly"), turn exponential.
And as for your following statement;
"Just to establish the simple principle that clearly escapes you, consider any human society prior to use of fossil fuels. When they (eg) burnt trees for energy, ... how much carbon (as CO2) was emitted that hadn't previously been absorbed from the ecosystem in the recent past? Hmm?"
Wakey wakey. This is the 21st century, and the majority of (the rest of) mankind has progressed beyond the hunter-gatherer society you describe here. If you wish to return to that sort of existence, that's fine. But I do believe you'll find that the vast majority of the rest of us have NO intention of joining you. And please, don't bother trailing out the tired old line of "If you don't change your wicked, fossil-fuel-burning ways you'll have no choice in the matter" cliche either. That's the trouble with all you eco-mentalists, its all "back to the horse and cart" and "Save Mother Earth". If you want to be taken seriously, propose a serious solution, ie one that won't result in the choice 'grow food or grow fuel'.