Caravan, Nobles (but the King, i guess) and Migrants can come from another Dwarven Civ.
But if you got a Civ that does not have a Mountainhome, there will be no liason - i am playing a map like this right now.
Yes liason and diplomats not being replaced is serious
More so since they tend to spawn far away from the Carvan and hence enjoy no protection from the guards.
Really annoying.
Anyway... In whatever fashion your liason got MIA - just forget about him. He won't come back.