I need a reasonability check for the safety of a home-hacked power supply
I think this is not a good idea.  You should use a 120V power cord attached to a fused power entry module http://www.digikey.com/product-search/en/connectors-interconnects/power-entry-connectors-inlets-outlets-modules/1442743, in turn connected to the right 5V/12V AC to DC converter rated for the currents you're interested in. 

Alternatively, just use a 12V wall wart with a high enough current rating and appropriate DC to DC converters or voltage regulators (linear or switching), with the whole deal fused.