as rami has already said, it is now tarmac to within 200km, then its a rough piste/track. they do grade it occasionally so could be good.but remember that over every blind crest is soft sand, there is a really good campsite in duenza[the village where the piste to TB2 starts.] cold
and good food.
as for visa and paper work, we got the visas before we left, bt i think you can get the mauri. at the border.
we had carnet,s but i have read tales about them not being needed now.
all i can say is that we were aked for them at the borders.
be careful crossing no mans land between morocco and mauritania.dont stray off the track as it is a mine field, from memeory follow it to the left to avoid the worst of the soft sand.
as for 28 days, yes it can be done but it will be tough, long days and it is africa, anything can happen and probably will.
we had a blast, i went the year the dakar ralley got cancelled, as we had planned to follow it, and still had no issues with securety.
if you decide to do it get in touch and ill see if i can dig the map out.
no gps or links im affraid just a route on a paper map and memories.
paul