These kind of problems are never easy.
All you can do on the ground is check for leaks while its running.
Not much help when the problem occurs after some time and possible affected by altitude.
The MM has Fault isolation section which provide the logical steps and the engineers will have already gone through that.
The only other point is you say there is a pool of oil under the engine. It should be possible to see where that is coming from but it may will be from a collector drain which several different areas are drained into one.
As mentioned the MM will have a maximum consumption per hour and that can be monitored.
The other thing that can affect it is altitude so the flights when it happens should be checked for this and power settings.
Been down this road and its sometimes a long one
