Final notes on this thread.
Found PCV valve to go "half-dead": it was rattling, but blowing to it it was letting air go both directions, very different from new one, so I replaced it at $7.
Drove 290 miles after all that repairs and settled MPG to around 17-17.5 per gallon, no SES light !
So, root cause for P2A00 was two-fold likely:
1: massive exhaust leak on the flange from the front catalyst to the back catalyst
2: PCV valve worn to the point where it was creating what ECU considered a vacuum leak