The answer to your questions is not "either/or" but "both/and." Take the following sentence (please!):
The incriminating evidence--the bloody knife with his fingerprints on it--implicated him in the stabbing.
Think of "to incriminate" metaphorically. In keeping with the etymology of the word (which means basically to accuse), someone or something accuses you of being somehow guilty. It screams out incriminatingly, "This bloody knife with your fingerprints on it proves you committed the crime!"
Think of "to implicate" metaphorically. Someone or something indicates you are involved in something shady. The word involved, in keeping with its etymology, means there is an interweaving of people and actions. By implication, this someone or something proves your involvement. It screams out, "This bloody knife with your fingerprints on it proves you were involved in this crime somehow." Involvement in a crime does not necessarily incriminate someone of having committed the crime, however. Perhaps the person whose fingerprints are on the weapon was an innocent bystander who pulled the knife out of the victim's body!
How, then, do we answer your questions? Well, a few scenarios could append to the three persons in your two exemplars:
Person A, the interrogatee, both voluntarily and unknowingly implicates his friends, B and C.
A piece of incriminating evidence, for example, slips out of his mouth (either accidentally or as a result of a skillful interrogation), thus implicating his friends.
Person A involuntarily--and thus unknowingly--spilled the beans, implicating his friends.
The implication is that the skillful interrogator, by simply keeping Person A talking, gets him to say something he would later regret. A Freudian might suggest person A voluntarily implicated his friends because he didn't want to bear the full weight of the crime, but either way, whether consciously or unconsciously, he involuntarily uttered the incriminating (i.e., accusatory) information. How much of a role coercion played in his utterance is unclear.
Can an utterance be both involuntary and conscious? Yes. If the coercion exerted on Person A is both unrelenting and skillful, he can be "broken down" and thus consciously, albeit against his will, implicate his friends. The utterance can even be untrue!
Police have been know to do a number of things that, while perfectly legal, make defense attorneys very nervous. The police are allowed to lie in an interrogation, for example. With friends A and B in a room that is separate from person A, the police could say to person A, "Your friends have already implicated you [the truth is, they have not], so you may as well tell us what really happened." So he does.
What might initially have been his conscious resolve not to implicate his friends turns into a deliberate effort to incriminate them, so that they all share the blame. Person A then says, "Ok, I did the crime, but person B drove the getaway car, and person C held the victim so that I could stab him." In this instance,
Person A voluntarily and knowingly implicated his friends in the crime.
Would his utterances have been as "voluntary" and "knowing" had he known the cops were lying to him? Probably not, but then sometimes in police work, the ends (i.e., the confession) justifies the means (i.e., lying).
In conclusion, the words implicate and incriminate do overlap somewhat; however, incriminate is the "stronger" word, carrying as it does the notion of accusation, whereas implicate is a word that indicates, by implication, involvement.
Lawyers distinguish between evidence that is exculpatory and evidence that is inculpatory, with the former indicating or proving innocence, and the latter, guilt. Similarly, incriminating evidence is at least potentially inculpatory. On the continuum of guilt and innocence, implicating evidence is definitely there, but just not as far along the continuum on the guilty side as is incriminating evidence.