the answers are not particular to the 747
(a) the concept of a critical engine in propeller aircraft relates to the change in direction of airflow through the prop disc and the resulting lateral movement of the net thrust line for each motor at takeoff pitch attitudes due to prop blade angles on the rising and falling blade side of the disc .. the motor which has the thrust line move outboard gives the greater yaw moment ... hence failure of the other motor (with inboard thrust line movement) causes the maximum control difficulties .. the latter motor is then referred to as the critical one because its failure causes the most control difficulty. With a jet, the engine is ducted from an airflow viewpoint .. so there will not be any movement of the thrust line ... and there will be no critical engine in a fashion similar to propeller aircraft.
Another problem relates to the normal (vertical) force associated with this airflow direction change .. and the similar force at the front of the jet engine .. but that is tied up with pitch stability rather than yawing problems.
(b) refers to the undercarriage assembly geometry on large aircraft .. multiple axle assemblies eg two axles on most larger main gear units.. and on some, even more . eg the 727-ski has a 3 axle main and 2 axle nose
(c) post failure planning depends on the crew's assessment of
(i) urgency to land
(ii) weather minima
(iii) local terrain and actual weather
(iv) available letdowns
(v) crew familiarity with the airport
etc ...
this assessment will lead to a decision whether to
(i) return via a visual circuit
(ii) return via a letdown
(iii) divert to a more suitable alternate