you have probably highlighted all the things to check in your post.. but...think on this... its a clue..:
1. unburnt fuel means its not being burnt.. its reaching the cylinder, not burning completely or at all, then being pushed out into the exhaust.
2. has the engine warning light come on? if not...the ecu isnt registering a fault from sensors , neither is it having a problem setting up the air fuel mix,
so i would use an obd reader to see if it could tell me of any faults that the ecu isnt flagging. you have a fault code-p0170
i would expect that unburnt fuel in the exhaust. to mess up the oxy sensor function.. and it will naff up the catalytic convertor. so not to see an ecu fault code displayed by it turning the engine warning light on, means that the ecu is still able to keep the air fuel mix ratio in range... thats tells one the ecu cant see a fault.. yet. https://en.wikipedia.org/wiki/Exhaust_gas
3. if there are no fault codes and the car " appears" to run ok.. then something that the ecu cant see, and cant indicate --- is happening... what might that be?
the ecu is trying to set the mix, its still doing that within its operating range, the oxy sensor isnt going out of range, the injectors, cam and valve control isnt flagging fault to the ecu...
4. its something after all the sensors?
spark plug not firing, even though the ecu is telling it to and cant see any prior ignition faults - that could be caused by a duff ignition circuit.. ie no high voltage generation, duff lead, shorted lead, duff plug.. all of those are after the ecu sensing..
i would check the whole ignition circuit, its connectors, supply, the coils, the leads, the plugs...
5.valve not closing properly.. not every time... but some of the time.. before the fuel has burnt... that could be difficult to investigate, because one might not see it with a static compression test, if it only happens at certain revs or at certain times..
6. valve not opening properly...not every time... but some of the time.. before the required amount of air for the fuel being delivered is the right mix.. an air intake valve problem..
7. - 5 and 6 are not common problems.. and usually are indicated by odd engine noises, and bad running, and idling.. even tapping noises on a regular basis if the timing starts to go way off... 5 and 6 are bad news, because the ecu cant see those faults.
your fault is more likely to be an electrical ignition fault, than a mechanical one.. given that you know the condition of the engine isnt far out.. just not meeting the spec..
and just one plug, being black and wet, not all four,,, is giving you a pointer which part of the ignition to look at.
8. if you are absolutely sure, that the ignition circuits are A1, but still get one black and wet plug.. it could be a cylinder problem, and you are getting oil into that cylinder, that is stopping the plug firing and contaminating the mix. i hope its not
as an aside.. a duff iacv will affect all four cylinders.. not just one. and the mix ratio wont be able to be set by the ecu, which should flag a fault... i.e it cant set the iacv.. it tries to.. but the mix stays wrong, and goes out of range..
you get a p0170 fault code because the oxy sensor is being contaminated by unburnt fuel.. and you have one wet plug not four wet plugs......
thats where the fault is. not the oxy(lambda) sensor...not the iacv... but the plug not firing.
9. if you clear the faulty plug ignition problem and are absolutely sure all four plugs burn the fuel properly, but still get a p0170 fault code.... then maybe the lambda sensor has gone awol...fried itself.. because if the air fuel mix is good, the lambda should burn of the contaminants, the exhaust should clear them out... and a good run should mean the car will pass its emission test..