Is a marriage contracted through an unregistered agreement with a lawyer and witnesses, then abandoned without official court documentation, and with subsequent details of fraud and deception, considered Islamically binding? And how can one deal with threats regarding a promissory note (amanah receipt) written as a deferred dower?

If the marriage contract is legitimate and authenticated by the court, then the woman is your wife, and it is impermissible for her to refuse to return to you. It is also unlawful for anyone to marry her until you divorce her and she completes her iddah (waiting period). It is advisable to appoint a lawyer to obtain the official documents proving your marriage and to file your case with the court. Furthermore, increase in supplication, turn back to Allah, and repent, for "whoever fears Allah - He will make for him a way out and will provide for him from where he does not expect."