Is it obligatory to seek out and communicate with old friends with whom there has been a long-standing estrangement and abandonment for many years, as a condition for the validity of repentance, and is communicating once sufficient to end the estrangement, or must communication be repeated, and is one sinful for not communicating with them?

You must end the boycott. Responding to the greeting is sufficient to end it. If you meet your companions and exchange greetings, there is no blame upon you. If you do not meet them, there is no blame upon you, provided you resolve to greet them and not turn away when you do meet them. The forbidden boycott is turning away upon meeting, and it does not occur if you are absent from them, given your eagerness to end the boycott. It is preferable to seek them out to inform them that the boycott has ended, even if it is not obligatory.