Typically I like to keep feed well away from nesting broody hens as that can attract predators by smell. Your situation overrides that, especially if bad guys cannot get in.
The temperature that matters most in this case is what the hen can ensure while actually incubating and how low the egg temperature goes when she leaves nest to replenish herself. I have had hen successfully incubate even when ambient temperature was below 0 F. Key to success hen spent very...
I would inspect clutch to make certain no more cracked eggs remain. If so, then remove them. Eggs that got too cold prior to incubation not likely to hatch. Hopefully your heat lamp prevented that. Even a clutch of bad eggs will be warmed well by broody hen only to not produce chicks.
Heat application can be important in preventing eggs from freezing prior to start of incubation. My hen above got lucky in having about two weeks of good weather as her clutch was being laid. A dominique hen parasitized her nest adding additional eggs during the same interval.
Game hen (technically a pullet) below covered more than 20 eggs with more than 20 hatching. Temperature got down into middle tens while she incubated. Nest location was on north facing front porch. No external heat applied.
Details in thread linked below...