Lennart’s Marin AB Safe House indeed wasn’t available for fast travel anymore (discovered it earlier and it was no longer on the map). It did show in the compass however and all the safe house stuff was present in the location itself.
The first Alpine Unrest mission did start automatically for me when I entered the game for the first time after buying the DLC. Lennart’s Marin AB Safe House was unlocked again when I reached the first safe house on the shore of the island. I have no idea if it unlocked earlier; I was too busy running away from the welcoming committee.
I did notice earlier that the first Alpine Unrest mission is registered on the Mården Bunker Warboard. Obviously I can’t test this myself but maybe you can try picking it up there manually.