@JT One of the most troublesome regs for us is 25.251(b), precisely because it prescribes a specific means of compliance, which if strictly, legalistically, applied, would result in having to flight test every last rivet change on the aircraft ....
Re the contra-rotation thing; I think it's true to say no normal modern design uses contra-rotating/handed jets, but I believe there may have been examples in the past. VTOL types, I think, may have been exceptions, too, but that's more for gyroscopic reasons, and hardly relevant to an ATPL interview.
@m_h That no engine will be critical on a jet type is also dependent on symmetry of the design, and the interplay between the engines and the control systems. If, for whatever reason, the type has more control power available after one failure than another (due to an asymmetric hydraulic architecture, say) then one engine may well be critical for some handling cases, including VMC.