Is it obligatory to return and endure humiliation, shame, and threats, or are repentance and seeking forgiveness from Allah alone sufficient to expiate the sin?

You erred in your action by allocating a portion of the funds for the sake of Allah, by withdrawing money from partners and others under the pretext of new ventures, and by not informing the partners of the true outcome of their funds. Your duty now is to repent to Allah and return the money to its owners whenever you are able, or seek their forgiveness for it. You are obligated to repay what you spent for the sake of Allah and what you took after the loss of the money. However, if any of it was lost before that without your negligence, you are not obligated to repay it. Ask Allah for assistance in repaying your debts by saying, "O Allah, suffice me with Your lawful provisions against Your unlawful ones, and enrich me with Your grace, making me independent of anyone other than You."