Which court has the authority to compel my ex spouse to pay or modify the existing order to make his loan payment part of alimony/child support?

There is more than one way to skin this particular cat, so it is best discussed "substractively." Nevada has no jurisdiction over him relating to property and debts, or over enforcement of the decree. Under UIFSA the ONLY court that can ever modify alimony is SC. You could either (if local rules there permit it) get the substantive ruling of default and indemnification in SC, and then move to domesticate and enforce it in VA, or reverse the order, start with domestication of the judgment in VA, and then move to hold him in contempt/seek indemnification there. Your first move should probably be a chat with your divorce lawyer in SC to see what the options are in the place that entered the original judgment.