Najafi Q&A: Debt

My father passed away, and I came to know that he owed debts to many people. Some of these debts are documented in writing, and some are not. If one of my father’s acquaintances came to me claiming that my father had borrowed money from him, but without written proof, and he says he is willing to swear on the Qur’an that my father borrowed from him—am I required in this case to repay the amount if he swears?

Is it obligatory to write a document that proves the amount of the debt along with witnesses, or is it only recommended?

Is the obligation or recommendation of writing and witnessing the debt conditioned upon the lender requesting the borrower to do so? And in the absence of such a request, is the borrower not obligated or recommended to write it?

Is the command mentioned in the verse: "When you contract a debt for a specified term, write it down" (Qur'an 2:282) a legislative (mawlawi) command or an advisory (irshadi) one?

A person borrowed money from the bank to lend it to another person because the latter does not meet the requirements to receive the loan. We have two transactions: the first between the bank and the first person, and the second between the first person and the second person. The second person pays monthly installments to the first person, who in turn pays the bank. The first person passed away, and the bank waived the remaining installments. Should the second person be obliged to pay the remaining installments to the heirs, or is their obligation waived when the bank waived the first person's debt?

Is it permissible to borrow money from banks with usurious interest for doing some commercial work in buying and selling homes, especially this is what is happening in America? Is it possible for my Christian partner to borrow such money in his name or in the name of the company with which I am a partner, and can I dispose of this money for the benefit of the company, then share the profit and loss?