Simply put, it depends on the circumstances, and the way in which they are handled. A PNF, SIC, or copilot is liable as a required crewmember for both enforcement action, and civil action. The degree of that liability will depend largely on the nature of the individual circumstances, the action taken by the SIC before, during, and after the event, the actions of the PIC, and the attitude of compliance shown by each individual involved.
In your first example, you describe the PIC losing control of the aircraft during a rollout. However, it would be useful to know the cause of loss of control. Mishandling of the aircraft, contaminated runway, animals on runway, crosswind in excess of aircraft or crew capabilities, incapacitation, inebriation, excess airspeed, flying with known mechanical deficiencies leading to loss of control, etc, will all be viewed differently.
In the case of a loss of control on landing which followed a poorly executed approach, the copilot may easily be found liable for the subsequent accident/incident due to failure to mitigate the situation. Or, the copilot may be exhonerated due to vigilence in attempting to prevent the accident (which may have occured due to no fault of the copilot). There are many variables.
The same may be said for a busted altitude. It's well to remember that even though the copilot may take verbal action in identifying the approaching altitude and in warning the captain/PF about the impending bust, the copilot is still a required crewmember, and may be held liable for the conduct of the flight. This may seem to be a technicality when discussing an altitude bust, but consider a landing accident. If the copilot allows the flight to continue into an unstable/unsafe/illegal condition, the technicality may turn to a fatality. The copilot's butt is tattoed to the same bit of real estate as that of the captain, and despite the disparity of their roles, one must ask how much the copilot values his or her own life.
If the copilot allows the captain to make an error which could have been prevented, the copilot will share in the liability. Consider again that there are many variables, and the degree to which the PNF copilot may be held to enforcement action is subject to a wide range of possibilities.
Captain lands on taxiway instead of runway. Again, many possibilites. What did the copilot do to prevent the event? Is it likely that the event was done innocently, such as a situation might occur during low visibility with heavy airport construction in progress? It's happened from small light private aircraft through major airlines, and it will continue to happen. Did the copilot object? How did the captain respond? If there was no response, did the copilot ask again, and failing a second response attempt to take control with the assumption of an incapacitated captain? What is the outcome? Many, many variables, each of which will weigh in to some degree to determine the outcome.
Runway incursion. How aware was everyone? It's possible that the event was strictly a controller error. It's possible it was a readback error on the part of the copilot, or captain, or both. Lost on the airport, and failure to get directions. Failure to adhere to a taxi clearance. Many possibilites.
Ramp check by FAA; no weight and balance completed. The PIC will get nailed, the copilot will likely be subject to enforcement action, the dispatcher will likely get at a minimum a reprimand and administrative action, and the company will be held liable. The degree to which this happens and the penalties given will depend on the circumstances, and the responses of each individual involved, including the inspectors.
Remember that the administrator isn't bound to impose a specific penalty, and under administrative law, you are guilty until proven innocent. Responses by individual inspectors will vary. Penalties will vary. Civil fines may be imposed. To answer difinitively is impossible. A determination will be made based on the ability of the SIC to intervene and prevent the occurance, and the degree to which the SIC exercised that ability. The attitude of the SIC, PIC, and the company will be taken into account, and most certainly the attitude of the inspector will have a bearing on the outcome.
Can a SIC be held liable for the actions of the captain? Yes, and in some cases, no.
