Is it obligatory to pay a debt owed to the uncle, verbally bequeathed by the deceased father 20 years ago, with no witnesses or documents, and with opposition from some heirs, and how should the father's estate and then the deceased mother's inheritance be divided?

You are obligated to execute your father's will by paying his brother's debt if it is legally established, provided it does not exceed one-third of the inheritance. Anything exceeding one-third is not enforceable except with the consent of the adult, sane heirs. The uncle's debt that is considered is what he owed at the time of your father's death. As for the division of the inheritance: his two wives receive one-eighth, his father receives one-sixth, and the remainder goes to his children, with the male receiving the share of two females. As for your mother's inheritance, it is divided among her sons and daughters by ta'sib (residuary inheritance), with the male receiving the share of two females. Inheritance matters must be brought before the Sharia courts for resolution.