A 150W heater will just fit inside the 180W fuse limit of both 12V power points combined. However, that electricity (like heated-seat electricity) will come out of the 12V battery, which I think will be recharged from the traction battery if the 12V battery--which runs all the electronics, controls, and other vital functions--gets depleted enough to risk shutting off the car. I haven't been able to find out the DC-DC converter algorithm, so I don't understand exactly what triggers that recharge and when it happens on the road rather than while recharging, but in any event, your plug-in heater's addition to your range will be trivial: you have ~19 kWh of useful charge in the 23-kWh high-voltage traction battery, so drawing 150 W to run cabin heating equivalent to your proposed plug-in heater for, say, an hour would use only 0.15/19 = 0.8% of your range—not worth buying the extra heater for. (Even if you can find one that small, too, it creates an unnecessary cord tangle, potential fire risk if paper gets near it, etc.)
My advice would be to preheat the car using MyFordMobile app's Charge Settings (set your GO Time and temperature target) while plugged into 240V charger, then use the heated seat(s). This works fine for me high in the Colorado Rockies where it was –24C the other morning. Indeed, now that my 2013 FFE, after ~500 miles, has learned my Technique, Temperature, and Terrain conditions, it often displays 75 miles' predicted range in the morning and has even gone as high as 79 miles despite our mountain midwinter...because it knows I'll leave the cabin temperature on LO (≤59F) and hence leave the cabin heater off. If I switch the cabin heater on, even at 60F, the predicted range immediately drops 10–20 miles; if I switch it back off, the predicted range returns immediately to its original value.
