I THINK I have similar issue. Here is my log of what has happend:
5/4/15- Bought car. 2 blocks off the lot check engine light, lack in power, and nasty noise from turbo.
5/5/15- P0021 & P0011
Result- Confirmed Bad Turbo
5/11/15- Changed Turbo, Coolant, and Oil/Filter
NOTE: Oil was black and filter was tough to get off (Don't think they kept up with regular oil changes)
Same Day- P0301, P0303, P0304
5/12/15- Changed Spark Plugs
Same Day- P0301, P0304 (Got rid of misfire in bay 3).
5/15/15- Took into mechanic
Supposedly they checked the injectors, coils, plugs, and MAF.
Still no solution, they told me it must be internal. However, it doesn't lack power or have any "engine shakes" (For lack of a better term". My buddy is a tech for Volvo, went though the Nascar program in NC. I had him check it out at Volvo and he couldn't find anything either. He says the way that it drives, he is highly doubtful that it is internal. Just stumbled on this thread and it seems to be very similar.
Let me know if you have thoughts on anything that you have learned in your process. P.S. your well documented process HAS helped me think through possibilities. I appreciate it!