What is the ruling on taking mixed funds from stocks to pay off a debt, knowing that I was in need and unable to take from my parents, and is it permissible to repay it by asking someone else to pay it on my behalf?

Unlawful money must be disposed of through charitable channels such as the poor, the needy, and paying the debts of those in hardship. It is permissible for you to take this money to pay off your debt if you are unable to do so, even if your father is solvent, because he is not obligated to pay your debt, even if your maintenance is obligatory upon him.