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your symptoms do suggest massively overfuelling - black smoke and misfire. if you've eliminated ignition (and assuming the injectors haven't started to misbehave) then you have a lack of air, compared to what the ecu is expecting.
do you have anything that allows a monitor of the AFM voltages? if one of the turbos is not working, i'd expect to see a big difference between the AFM outputs when you go on boost. you say the AFMs are new - are they a direct replacement or have you an ECU that allows differences (and therefore have been programmed accordingly)?
i'd have thought that if a turbo "sticks" with turbine and compressor wheels complete, the restriction that would cause significant running problems even before transition btwn vacuum and +ve pressure (since 3 cylinders would be receiving less air than the others). i imagine the force of the exhaust gases is quite high and the standard oil film bearings fairly robust, so can't think this would happen. if the ceramic wheel were to come off, it'd be left in the exhaust housing. since nothing is driving the compressor end, that would spin due the air being pulled be induction. idle would be ok but as pressure increases, since only one turbo is supplying compressed air, there will be a point at which the consumption becomes less than demand (the transition point) and air flow would take the easiest route and (might) reverse the flow back out of the knacked turbo. again, the AFM voltage would clearly show this.
if you're happy all the pipework is sound, i'd think the only thing left to do is remove exhaust elbows and check the turbines. i don't think your problem is electrical, symptoms sound mechanical.
good luck,
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