Will the father be sinful for dealing with usurious banks based on the fatwa of the Egyptian Dar Al-Ifta permitting it, given his belief that it is not forbidden? And is it permissible for the son to marry using mixed funds, some of which are from usurious interest provided by the father, given the son's belief in the prohibition of usury, his need to marry, and his inability to cover the expenses himself?

Ask Allah to guide your father and grant him the ability to follow the truth. Your father's sin in adopting the permissive opinion depends on his assumption and trust in the mufti. If he believes that the mufti is issuing a fatwa based on truth, then we hope he will not be sinful. However, if he adopts the opinion out of personal desire and inclination, then we fear for him sin. You have done well in advising your father, and it is recommended to seek the help of scholars to convince him. There is no harm, Allah willing, in your use of your father's mixed wealth for marriage expenses. However, if it is possible to do without it, that would be better, because the majority of jurists consider it disliked to deal with someone whose wealth is mixed.