Well, well done Arnold’s new family! I can completely understand your frustration as we had a similar thing when we adopted Dora. She’s luckily not as bitey as Arnold but she was about the same age and similar background and all I can tell you is she’s much happier now and rarely nips... (only to say, ‘oy!’ Rather than meaning, ‘Grr, go AWAY!’
Apparently she regularly drew blood in her old home but only nipped me twice. She took to her bigger cage brilliantly and will even be held and stroked now... up to a point. Keep going. It’s taken us four months of many hours with Dora but what a happy little personality she is now. It’s worth it.