Both positive and negative half waves go together, if one is missing, the other one will be too (except rare and improbable situations).
Go with the drawing you have, write the sequence detecting program for it and try the various situations you think of and see how the program behaves.
BE AWARE THAT THE VIRTUAL GROUND (or Neutral) WON'T BE 0V ANYMORE IF ONE OR MORE PHASE IS ALTERED OR MISSING. DO NOT TOUCH !!!