1. Yes, I would interpret ferm as farm.
2. Regarding the birth order, I think he has two sons named John.
John is the eldest, then (in some unknown order) come Robert, William & Edward, followed by John the youngest.
He is careful to qualify any mention of a John by adding eldest/youngest (unless referring to one of John's children, which might not be ambiguous).
ADDED:
Although, on looking again, it's possible that the references to eldest/youngest are relative only to the Johns, and not to the other boys.
I don't think we can draw definite conclusions, beyond that there were two Johns.