Abigail cant read so I guess she can't write
Charles was in Canada and sadie was in South America so I guess Ross and the bureau taking John's family prevented him from asking for help either that or John didn't wanna risk them dying after what happened in American venom
Jack in the epilogue was on the same level as rdr1 John who was better than Arthur despite only being 19 so I voted Jack
Since rdr1 was made before sadie and Charles didn't exist but I guess story wise the bureau didn't allow John to communicate with them
It is possible but u will have to repeat the train robbery mission each time