Rupert immediately leapt to mind! I'm not a fan of Archibald mainly because of the Bald portion, and I do think Archie is a bit matchy with Charlie, leaving Iver rather out in the cold. I also love Gerard for you, I think it has the same vintage quirky feel and it's similar to George but less traditional.
Otto would be ace! Or Viggo! Good, solid Scandinavian names - Anders also is lovely, Thor (pronounced Tor) is super cool.
John, nn. Johnnie is underused and totally ready for a comeback!
Solomon nn sol
Harold nn Hal
Malcolm nn Mal
I have a cousin called Archibald, nn Archie - it's a fantastic name and so substantial! The nickname is cute on a little boy and translates well into adulthood. If he really wants when he's old, he can still get the nn Archer from it - very simple!
Edward, nn Ward, or Woody
Edwin - Win!
Thomas - Tommo
Alexander - Xan, Skandar, Sasha
Alasdair - Ally
Lawrence - lawrie, lazslo
Christopher - Topher
Rupert - rudy
William - Billy, Wiley
Joakim - joe
Brennan - Bren
From your list, I really love George, Arthur, and Walter. All were on my list for this baby early on :)
Martin (we're using this, with nn Mars)
Richard nn Hardy (I feel like this could split the difference between Iver and Charlie)
Edmund nn Ned
Frederick nn Fritz
Francis nn Frankie
Hmm, I know I had more, but lost my train of thought!