If you are thinking of crossing in July then Rosso and Diama are the only really suitable options. By July the river level and water flow has increased leaving any other options dangerous. I'm not aware of any place where it is possible to ford the river, as it is too deep and too wide in most places. Depending on the amount of rain which has fallen in July the Diama route may be impassable too. Although it is possible in a 4WD the piste is sometimes closed by the police to stop it being too badly damaged. It used to be possible to take a smaller ferry upriver but I understand that it is closed now. Anyway you would have problems with the paperwork for the car if you didn't arrive at a recognised crossing. Not having valid papers for the car means that you could easily lose it if checked. If you have time and are interested you can drive through Mauritania and head south then southeast from Aleg and drive into Mali, cross the river in Kayes (bridge), and then head into Senegal. I did most of this route on foot and with an old Chinese bicycle about a year ago and it was well worth it. I can guarantee you you will meet **** all tourists but I suppose seven Landies will cause enough of a commotion anyway. If you are thinking of bolting the Landies together then I suggest you stack them on top of each other, at least the guy on top can see where the hell youre going to.