If I could have talked to him, I would have told him not to come. I knew that the conference would be/was going to be very boring. :)

If I could have talked to him, I would have told him not to come. I knew that the conference would be very boring. Could have, because I am describing something that could possibly have happened in the past, but did not: third conditional. Would be, because I am describing the future from the perspective of that past time: second conditional.

Has the conference already happened? If I could HAVE talked TO him, I would have told him not to come. I knew that the conference would BE very boring.