If you change the heat conduction of one particle, it changes it for all of them. You should never modify ptypes individually for each particle.
Find this: "//heat transfer code" in update_particles_i in powder.c, and then add something like this: int hconduct = ptypes[t].hconduct;
Then just replace all ptypes[t].hconduct's with hconduct and it should work.
if (t == PT_INSL == parts[i].tmp == 2)
hconduct = 2;