I need an informed opinion about what a dealer's "Service Adviser" (aka
hustler) is telling me. The check engine light went on
on my 1999 E430 with 75,000 miles and they said it was
the Mass airflow sensor and they replaced it. One day
and 40 miles later I was accelerating through a turn
and the check engine light came on again and engines
shook so badly it felt like it was missing 3-4
cylinders. They now tell me my catalytic converter is
shot and the o2 sensor could not get a good reading. I
suggested that I have never heard of an O2 sensor
having such an effect on the fuel delivery system as
to completely cut fuel to a few cylinders, he then
backed away from that diagnosis and said it was back
pressure not allowing enough air/fuel in to some or
many of the cylinders. An aftermarket catalytic convert maker tells me I could possibly cut out the pre-cats and simply replace the main cats if they are even bad. I would be inclined to do this and to save $2500 if there is no conceren for a particular level of back pressure required at that point in the pipe. This vehicle has two O2 sensors per side, one in front of each pre-cat and one behind. Does anyone have an informed opinion regarding the technical viability of this kind of solution? Thanks.
P.S. I can't help but wonder if anyone wacked these units the day before with a rubber mallet ?

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Problems while owning '99 E430
Continual ABS/BAS problems.
P.S. pre-catalytic convert rusted out at 90k
Ignition key actuation problems.
Gear shifter shift gate issue.
Bad antenna amplifier.
CD changer.
Heated seats never worked.
Biannual air conditioning failure.
Power windows and moon roof.
Leaky valve covers.
Rear power window broke.
Poor rear lamp connections due to bi-metal corrosion.
Maybe my spring perches will fail someday!
Engineered like it's not a car