Why only L.H. failures?
Surely if this left hand air supply theory is correct then it can only be a matter of time before a cabinful of S.L.F. become incapacitated, if the R.H. system is the same as the L.H.
However, if the L.H. system differs from the R.H. system, i.e. because one (main cabin) has to be of greater capacity, then the difference should provide a clue to the cause.