What if Arthur went to Copperhead Landing with Abigail and Sadie?
While I can’t see Arthur just giving up on Dutch and the gang, especially with the information that Micah ratted them out, I can see him making Abigail and Jack’s safety his main priority instead. With Arthur thinking that John’s dead, I think it’s possible he would dedicate the time he has left to making sure they can start fresh somewhere. This would allow Arthur to live slightly longer too. While he did succumb to his tuberculosis, it was the exhaustion from his fight with Micah that sped up the process.
The whole Beaver Hollow situation would actually go down about the same. John would get into a standoff with Dutch and Micah over them abandoning him and the Pinkertons would still attack. John would likely be killed or arrested during the shootout, but even if he did escape, he would have no idea where Abigail and Jack are. Maybe he would eventually be able to find them though, in which case the epilogue would probably happen in the same way.
In the scenario where John does get reunited with his family, I don’t think Arthur would stay with them, just as Sadie and Tilly didn’t. While low-honor Arthur says he plans to go somewhere dry to wait out his TB, I personally don’t believe that’s canon. I think high-honor would try to find Dutch again to tell him the truth about Micah, as that was ultimately his final goal. That being said, I’m interested to hear others’ takes on this what if.