Do you think Charlie is feeling anxious about Amber's arrival? I've looked at your previous posts and can see that Amber has only been with you for a few days, that she's only 6 months old and that when she and Charlie play together he barks a lot. Do you think the barking is just playfulness or does it sound anxious? My older Cocker doesn't play with my younger one but he occasionally has a few seconds of running about with him and barks in a sort of hysterical sounding way while he does it. I think it's because he's unsure about being so close to Louis!
Even though Charlie's playing with Amber he might not be comfortable about her presence at the moment. It's a huge change to his life and it will take time for him to adjust. I think his running off on the beach and not coming back could be because he's feeling confused and anxious. He's not meaning to be naughty but being in a place where he always gets excited might be giving him a sort of adrenaline rush that he just can't control at the moment. I'm not sure if I've explained that very well, but hopefully you'll understand.
It would be a good idea to do some of their walks separately for the time being so you can do some training with Amber and just get her used to being walked on her own as well as with Charlie.
It can be really difficult for an existing dog when you introduce a new one. Even if they seem to get on well it's still a huge adjustment for the existing dog to cope with and their behaviour can change, hopefully just temporarily. Are there places, less exciting in Charlie's mind than the beach, where you can try him off lead for the time being? If it's somewhere that doesn't normally make him hyper you should have more success in keeping him close to you.
Good luck

. It may be difficult for the moment but hopefully they'll continue to build up their relationship and Charlie will settle down again soon.