How large/extensive would the Japanese civilian involvement be once the invasion begins?
How plausible is the idea of millions of Japanese civilians engaging American troops on the battlefield or through guerilla warfare?
The vast majority of the 30 odd-million civilians mobilized by the gov't would still have stayed behind to fulfill industrial and agricultural jobs, they were still the backbone of the country's workforce and would be needed to sustain the Army in the field. When the time came (ie, when their geographic region was threatened) they would be converted into logistic, combat, and partisan units to take a more active role in defense. The point of grouping them into People's Volunteer Corps was to essentially ensure that the GHQ in Tokyo would have absolute control over the main body of its adult population so that it could do as it saw fit with them; though this didn't exclusively refer to the fighting alone.