Nice soffit vents you’ve got there. They sealed then up nice & tight with the cellulose insulation.
Any ice build-up in the attic is signs of warm air traveling upward because of its natural buoyancy. As it reaches the ceiling in the top floor, it seeks ways to rise even higher through cracks and gaps in the ceiling and walls. Some of those pathways are obvious; many others are not.
Those openings around and through recessed canister lights, whole-house fan installations, attic-access hatchways and pull-down stairs, and electrical boxes in the ceiling and uninsulated walls all provide conduits from the house into the attic.
Additionally, heat is conducted upward through the top-floor ceiling through inadequate attic floor insulation, which doesn’t appear to be the issue here.
With that radon exhaust blocked as much as it is, there must have been some high winds in that area to cause that type of blockage. I hope you wrote that up as a safety issue. The radon fan is definitely not working properly, if that exhaust pipe is clogged. If the fan was operating properly during the snowstorm, you would think that the air from the fan would have blown this snow accumulation away. I’ll bet the fan was inoperative before the storm. I know it’s not in our SOP, but did you check the radon vial that indicates proper pressure?