Maybe when you won’t get sufficient answers here, aks your problem at help.openstreetmap.org … I think this is a general question that is interesting for all users.
I would add that you should use method 1 only when you know how the boundary is legally defined. In my part of the world, I come across many admin boundaries on the map, but I have no clue if they’re defined relative to any physical objects. In some regions, the admin boundaries are fixed by the original definition, and do not change if a highway or river is rerouted.