If you have completed all your terms and conditions of sentencing and have been compliant with probation, then you should never sign an extension of probation. Why should you have to extend your probation due to the fact that the probation officer can't do his job. The PO either has to file a petition to revoke you before your probation ends or they lose jurisdiction. If you don't sign the extension you tie the hands of the PO. If he wants to file for a request for a hearing with the Court, then fine. Present your case at that time. You should submit proof of everything directly to the Court now, so that they have proof that you are in full compliance. You may also request a hearing on the matter, but I'm not sure it is necessary for you to request a hearing. Again, you better be sure that everything is done and you have not done anything that is a violation. Bring proof of everything with you (in case the PO doesn't or claims to have not received it from you) and understand that your PO is not going to be happy.
My decision to answer your question does not construe an attorney client relationship. My opinion is based on the facts you have provided. Before making any decisions, you should always consult directly with an experience attorney, either in person or via phone.
The judge can grant you off probation but if the probation officer comes in with a good reason to allow the extension, the judge might just go with him. A lawyer is not a bad idea here.
I agree with Rhidian Orr. Unless the probation officer can get you to consent to extend your probation, or unless the probation officer can get the Judge to extend your probation, it will terminate at the end of the probation period. Even though your probation officer is in a county different from the one you were sentenced in, you will go back before your original sentencing Court. If the probation officer tries to take you back in front of your sentencing Court to extend your probation, then you can provide all of your proof of completion and ask the court to terminate your probation on time. Wait and see if the probation officer tries to file something with the court to extend your probation and in that event, I would consider hiring a lawyer to insure your rights are protected and to help you show the Court that you should be allowed off of probation.