If B did good job, you may want to pay him and let him finish the rest of the project. Since he is satisified, he might be willing to be your witness. Wait until A resurfaces, you can file a complaint against A using B as your witness.
If you don't want B to finish the project, you may still want to pay him partially to promise some installment payments for the reason that A may come back some days. This is a strategy plan worthy a try. Then find another contractor to finish this job.
At least in this way, you get it going again without any law suit or lien against you.
