Each card that you have on the table must be considered seperately. The rules are clear that phase actions can not be combined and cannot "accumulate".
For
Black Marshal, having 4 copies out will not be any better than having 1 copy out, as you get the same phase option. The only use of multiples would be to combat condition removal. In other words, you can only use the effect once - you can exert a Nazgul and play the other minion to add (1) or (2).
The same thought goes for
Bent on Discovery. You much treat each copy as a seperate card (cards can't be "morphed" into one, in other words) and meet the requirements of each one respectively. If you had 4 copies, you WOULD have to place 4 exertions - if you had 3, you'd have to do 3, and so on.
It's pretty simple really.

Hope this helps!