Yes ...like I said, artists can get close to the likeness of someone. But you still have to deal with people interpreting the likeness as observers. Different people will have different opinions, thus they will have different interpretation. Which then makes it subjective.
Here you have different people trying to interpret a portrait's likeness. An artist can come closer than the others, but you will still have different people with different opinions interpreting the portrait's likeness. Thus this is is subjective.
Painting is a subject of its own, yes. But here, it matters because it can change a portrait's likeness. And interpreting the likeness of a portrait when painted is also subjective. That is your opinion that the P1 Joker looks more like Heath Ledger than the Hot Toy's version. There are others that will interpret it differently and disagree with you.
Yes, one witness will be right and the other witness will be wrong. That's because both have different opinions about how the perpetrator looked like. That alone says interpreting likeness is subjective. Even if there are objective physical realities to use as guide, it still leaves to interpretation by the observers. And that's what's happening here ...we have different people trying to interpret likeness of a sculpted and painted portrait. It's subjective.
In the case of the two witnesses. If they're identification of the perpetrator is inconclusive, then investigators will need to use physical evidence from the crime scene to identify the real perpetrator ...like fingerprints, DNA, etc. Because only that can say which of the two witnesses is right.