I've been researching and analyzing cases of oil seeping through compressor-side piston seal. There are a few scenarios that don't include any damage to turbo itself:
1. Restrictive (for any reason) air filter causes excessive vacuum in front of compressor, and sucks oil out. The piston rings are meant to hold air pressure from outside from reaching inside of turbo housing, but are less effective the other way. They say.
2. Excessive crankcase pressure (for any reason, such as clogged PCV) does now allow free-flowing oil return to get back into the sump via return hose. That results in oil pressure inside turbine housing and easiest way out is piston rings (e.g. turbo seals).
3. Clogged/bent oil return lines, same effect as #2.
4. Excessive DPF backpressure due to it clogged, and potentially overcomes the seal

Now, in my case, we confirmed that the only issue with the turbo is damaged turbine wheel resulting in space between piston ring, potentially/likely allowing exhaust gasses into turbine housing.

Setting aside reasons for damage for now, the following is likely/possible with that damage:
A. Exhaust gas pressure leaked into turbine housing, from there, via oil return line, into crank case, increasing crankcase pressure. See #2 above.
B. Exhaust gas enters from turbine side and pushes oil to the side, while oil is pushed by pump from the top. That is enough to overcome the "oil slingers" (the round grooves). After that, piston rings are not able to hold oil even at negligent pressure and oil ends up getting past the ring, and from there into clean air side of the turbo.

Now, how did the compressor wheel end up getting worn/damaged on turbine side?
I can only suspect compressor surge (turbo flatter for brits). Compressor surge results in violent shaking of compressor wheel and shaft, radial, but nothing prevents axial movement, which would transfer along the shaft to turbine wheel.

The wear mark on the photo looks as if the turbine wheel was pressed against turbo housing and rotated, causing the sharp edges to spike up. It may very well be possible that radial or axial movement of the shaft was enough to pin the wheel against housing and cause that wear/damage.

Just thinking.

Once everything is assembled in a couple weeks, I will certainly go perform crankcase pressure test as first thing.