I agree, if King Charles chooses not to attend, I would think Anne would attend besides being Haakon's godparent, both have a very good relationship.
Harry, attending, representing the King? I think not whether Harry thinks so. He's never been anywhere close or really in touch with the Norwegian Royals and he's a non-working royal who has not carried out any official Royal duties for several year, (although he thinks so). In addition, he wouldn't be well received by the public and the UK public would be in an uproar.