To what extent is it forbidden to hold a mixed wedding party that includes musical instruments and forbidden songs, drinking alcohol, and young men dancing with young women, especially if those organizing the party are religiously committed and are aware of the unlawfulness of these actions?

What you mentioned about the mixing of unveiled women with men, their dancing, listening to songs and musical instruments, and drinking alcohol at parties, all constitute major sins. The sin of these abominable acts falls upon those who commit them, those who invite to them, and those present who do not denounce them. So, whoever does not avoid the company of those who commit sins has condoned their actions, and condoning a sin is a sin. The hosts of the wedding are complicit in these sins for gathering people for these destructive acts and preparing the venue for them.