You think it's possible to put any decent percentage of our GLOBAL food production in greenhouses (remember with less light global temperatures go down) within ~6 months?

Billions would perish. If the luckier rich countries did not get nuked or invaded by armies or waves of endless starving refugees then they would be able to save a good amount of their population. At best world development goes back ~50-100 years. At worst, modern civilization basically ends from the combination of conflict and famine.

You don't have the _slightest_ idea of how much energy and materials you would need to provide sufficient grow lights to feed humanity right?