As a cargo plane, my thoughts were how likely would it be for engine debris, admittedly at high speed with a lot of spin, to penetrate the fuselage (easy, thin aluminium) and the cargo (less likely?)
There is no real difference between a cargo and a passenger planes construction. Some cargo planes do have a strengthened floor to take the weight of pallets and extra loading doors, but the wings, skins and bulkheads are the same. I don't think the cargo has any bearing on this, because for parts of an engine to hit the cargo they have to travel up, so they will miss the other engine if they emerge out the other side of the fuselage.
AIUI there are contained and uncontained engine failures. Smaller things like individual engine blades are supposed to be contained within the engine cowling if they break off. The engine will be destroyed, but things shouldn't fly out of the sides. This is tested during engine certification, and is often implemented by having kevlar bands around the engine. Larger parts - like rotor disks (either whole or segments) are considered to have infinite energy and cannot be contained. If they do break off, they will go through virtually anything. The safety mitigation for these is simply to route critical wiring and hydraulics out of being in direct line with the high energy rotating parts. Engines are also mounted forwards of the front wing spar, so if anything does fly off it doesn't puncture fuel tanks.
If some high energy part of #1 did escape and somehow hit #3 it will be the first known incident of this happening. It will have very serious consequences for air travel. Aircraft are certified to be able to takeoff, fly and land on n-1 engines on the basis that it's highly unlikely that two engines can be damaged by the same event. If that assumption proves to be false then it opens a huge can of worms. n-2 isn't possible on any aircraft, and since almost all are now twins (B-737,767,777,787,A-319,320,330,350) n-1 means zero.
