Lplates is absolutely correct, the primary reason for the manual offset button is for the case of double APM failure or triple air data failure, both of which are pretty unlikely.
If the OFS captions are showing on the VMS it makes no difference to manually press the OFS button. It might be appropriate to use manual OFS when landing offshore through turbine exhaust (Ninians etc) but personally I have never had to do that.
On the CHC event, the accumulator should hold hyd pressure for many applications of the wheel brakes and rotor brake. That it didn't could be explained by a few possibilities, such as:
The crew attempted to lower the undercarriage using the normal method;
The accumulator one-way valve was leaking badly;
The accumulator gas pressure was severely depleted.
At this stage I have no idea which if any of the above applied.
By the way, it was low level, not low pressure (never believe what you read in the press!). Low level detection causes ancillaries to be isolated in an attempt to contain the leak and preserve fluid for flight controls.
HC