Can would have + V3 be used to indicate past possiblity

It is a bit difficult to understand exactly what you're asking (some provided examples within your question would be wonderful), but I think you're mainly asking: does "would have" strictly imply that the action did not happen, or could it be used if the speaker is unsure if the action happened or not?

"Would have" as a conditional

For example in the sentence, "The school would have had a ceremony if they had the funding," the speaker knows that the school did not have a ceremony because they did not have the funding. The sentence would be changed if the speaker was unsure about the ceremony. "The school might have had a ceremony if they had the funding," is a sentence from a speaker who is unsure if they school had a ceremony; it is also implied that the speaker did not know if the school had the funding.

The above shows "would have" being used because the speaker knows that the event did not happen, in the same context if the speaker is unsure if the event happened, the phrase is changed to "might have"; however, "would have" can be used in a scenario where an event could have possibly happened.

Consider, "If the driver took 5th avenue then he would have turned right, otherwise he would have turned left." In this sentence, the two events are mutually exclusive and the driver did indeed take one action or the other, so in this case "would have" is being used to describe both an event that did happen and one that did not. The speaker could have also said, "The driver might have turned left or right, depending on if they took 5th avenue." Both sentences convey the same message, but they are written differently to correctly use "would have" or "might have".

"Would have" as the past tense of "will have"

Speaker talking in the present: "I know I will have to get to work early tomorrow."
Speaker talking about the past: "I knew I would have to get to work early the next day."

The above shows "would have" being used not as a conditional, rather as the past tense of "will have," in which case it is possible the event could have happened.

Conclusion

So, to answer the question: "Would have" can be used as a past tense of "will have". "Would have" can also be used as a conditional, and whether or not the event in question could have possibly happened is more dependent on the wording that the speaker chose to use and the context of the situation, not strictly based on the usage of "would have."