You can find more information about acting on behalf of a minor in the following thread:
As I understand it, yes, regarding the subscription, they just want to ensure they get your money immediately, and not sometime in the future. Whether you are then indebted to someone for that money is not the concern of the IPO organizer. Nordnet (or your bank more generally) may, of course, have a different opinion on everything you are trying to get credit for.