Just back from a trip along the atlantic-route to Gambia in two mercedes 2WD: 4 adults, three small children. The road from Nouadibou to Nouackchott in Mauritania is almost finished. 15 km of piste left. Routefinding at border bewtween Morocco and Muaritania is difficult. Almost everyone gets stuck in soft sand there, and then some Mauritanians appear to help you for 50 euro.
Between Nouadibou and Nouackchott we left the tar 200 km after Nouadibou. Then drove cross-country to Ras Tafarist. Quite adventorous, but no real problems. Then we crossed the Azzefal-dunes on the route described in Sahara-overland. Not advisible. New dunes, and one steep descent. We were digging quite a lot. Then took the beach piste for 50 km, with water quite high. Not really recommended.
Auberge Sahara is a nice place to stay in Nouakchott.
No problems in Senegal with carnet.