rolling resistance would be hard to determine...deep snow, ice, wet roads, dry cold roads...Yes, rolling resistance in snow and rain is higher making your mileage worse. To go with that are the slower speeds you should be travelling at which again are another hit on the mileage.
Also if your traction control is coming on all the time (because you have all seasons and not winter tires) your both wasting gas and brake pads.
No, opposite would occur in slippery conditions, the transmission will shift sooner keeping the RPM's down to prevent excessive wheelspin. Transmission doesn't care what the engine temperature is regarding shift points.
If in (deep) snow, you should have DSC off, *and ideally transmission in Sport or Manual mode
*disclaimer, watch out for hidden obstacles under the snow...