How do we know that the sun is immune from such solar activity and will not have flares of such magnitude?
http://www.spacedaily.com/reports/Did_A_Massive_Solar_Proton...
For the old folks like me - there's roughly a 1:100 conversion between sieverts and rem. So an anticipated 3-6 sieverts over 2 days is 300-600 rem, which is fatal to nearly everyone.
I think the effect of the direct radiation would indeed be blocked by the typical roof and walls. The part where the prepper has an advantage is dealing with the indirect effects -- such bombardment would (temporarily) collapse the geomagnetic field and fill the uppermost reaches of atmosphere with dust. That would cause massive effects on the climate.
The amount of dried, preserved, high calorie, long shelf life food that we have staggering. Crackers, soups, noodles, pasta, dehydrated soy/meat etc.
The only two reasons someone could starve to death is if companies deliberately hid supply of food for fun, or if they refused to eat those types of food for about 60 days, upon which most people die from acute starvation.
Malnutrition is more complex but vitamins exist.
In terms of total global food production, meat production is a net drain, that basically consumes food to produce a luxury.