Do you think that only one aircraft manoeuvring is enough to create safe separation
Yes. Do the sums. The alerting times, and changes in flight path, are sufficient to generate a miss if the threat aircraft does not manoeuvre. Hence it is adequate against Mode C (non TCAS) threats.
Which is why, in most coordinated encounters, both aircraft can expect the RA to weaken well before clear of conflict, because safe separation is met with time to spare and the aircraft can be directed to paralled their original flight paths, thus minimising their deviations and reducing the chances of a third aircraft above or below becoming involved.
Furthermore, it is true hat I dont know everything but I never heard inflight RA inhibition (without warning the crew that they dont have RA available), since it is the last safety net we have.
They get an RA, but it'll be preventative.
Same thing happens in reverse in a low level encounter.
pb