Is it true that aircraft with all engines mounted at the rear could in practice be treated as centerline trust ie the engine thrust lines are so close to the longitudinal axis that any assymetric effect is negligible ?
Absolutely not! If that were the case my company's jet engined productes - which are all rear engined twins - wouldn't need to worry about minimum control speeds, and I assure you that we certainly do!
One important reason is that acceptable minimum control speeds (for performance limiting reasons) are an important sizing criteria for the rudder, and sometimes other surfaces as well. Since the rudder does nothing useful in 99% of flight, one will make it as small as possible to meet the design requirements.
So, while an underwing twin with engines magically moved to the rear fuselage position would undoubtedly see an improved Vmc as a result, a rear engined twin of similar performance class *designed as such* would have a rather similar Vmc to the underwing-engined aircraft, as it would (simplistically) just have a smaller rudder too.