I've been having virtually the same problems with my X3 as well. I too have replaced the elbow boot, DISA valve, and entire CCV system and hoses.
In addition, I've also replaced gaskets for the valve cover, intake manifold, throttle body, oil filter housing, oil dispstick o-ring, O2 sensors, spark plugs, even coils, and a few other seemingly unrelated bits and bobs. Took it to an indy shop for diagnostic and smoke test and they couldn't find anything helpful... (granted they ended up being a pretty shitty place after several visits).



My case might be a different mess than yours, but so far I haven't been able to track down the cause of these damn codes, even after tons and tons of researching videos, forums, car-FAQs, etc. My fix doesn't seem to be what everyone else has had work. For a while too, one code actually went away and I was left with just the P0171.


Next up on my "To-Do" list will likely be checking and/or replacing the MAF, EGR valve and hose (since it's part of the secondary air system that plays a large part in the vehicle's emissions), and possible exhaust leaks that maybe I'm missing...

**One thing I did notice that I would've never thought to check was the fuses. Was rooting around the fuse box in the glove box for another issue, and realized that 2 of my fuses (of the 4) related to the engine control was missing! (Apparently this is something shady sellers can do in order to keep the check engine light from coming on or hiding things of that nature). Could be something as easy as that...going to buy some to test it out.



I've also read few bits of information here and there about possible clogged/dirty catalytic converters, leaking brake booster (less likely though), and even an issue with needing to update/reset/replace the vehicle's PCM/ECU (powertrain control module/engine control unit).



Not sure if that helps out much, but it may give you some direction if you're at a bit of a dead-end. Seems like I'm going down a rabbit hole with these codes, by this point the car's gonna be brand new! I have read that, oddly enough, O2 sensors are usually a last-resort replacement for these issues. So if you can, start with the cheap and easy fixes first and check off from there.