I think the router idea is not good as it’s not really a backup. The router is just joined to the Neth AD so if the Neth AD fails the router is no real backup.
You may use Hotsync to get a slave AD server.
As regards planning, for Neth 8 the discussion is open.
I remember that the second DC approach worked recently but I can’t find the thread… 
Right, it’s not the case as the remote Nethserver is no DC.