One problem with using human waste ("nightsoil") as a fertilizer is that unlike animal waste ALL of the bacteria &/or viruses in it are human adapted and fecal contamination of soil/food products with human waste way more likely to spread serious diseases than using animal manure. There are processes to render human waste/municipal sewage suitable for use as a fertilizer, however in this situation the technology to do so is not readily available. In fact such plants may not have existed at all at that time in Nebraska, and if they did were likely in proximity to large cities (AKA targets).
As far as the radionuclides, it varies a lot. Some have relatively short half lives, others very long. And, of course, it depends upon whether they are alpha, beta, or gamma emitters as to the exposure risk externally (and also to some extent internally). Those nuclides which might be concentrated in plants, and are used in human metabolism (I-131 for example) represent the greatest risk as small doses in the soil get concentrated in plants and further concentrated in the body.
As far as "forced labor" goes, in a situation like this its no work=no eat. And, believe it or not, it is much better psychologically for the refugees to be doing something than sitting around in camps doing nothing. WW2 POWs were report that being locked up with nothing to do/boredom was often a major problem (of course this depended on your rank & who captured you). BTW almost anyone can do productive work - you don't have to be in the fields. Craftsmen can be used of course, "home economy" like sewing/knitting etc can be used to stretch the supply of goods which are no longer replaceable/disposable, grunt work not to physical around camps such as hospital orderly, cooking/KP, child care etc. Only the truly infirm or severely ill need be excused...there will be plenty of USEFUL work to be done.
