I think Dutch wouldn’t have gone to the “truce” meeting with Colm O’Driscoll, and Sean probably wouldn’t have died when he did, as the “job” they did was mainly because of Micah manipulating Bill into it and the latter not being sharp enough to notice. Also, the Pinkertons may not have found the gang at Lakay and may not have found Beaver Hollow in chapter 6 either; the gang standoff would never have happened.
In terms of the gang, it’s hard to say if it would’ve fallen apart or not. I suspect John would’ve left and Arthur may still have helped him, but all the main remaining members except for Charles may well have stayed with Dutch. Arthur still would’ve died, due to his TB.