Our recent experience is that you don't need the carnet for Niger. However, arriving from Gao, you cannot buy insurance at the border, but only in Tillaberri. We, and another car, were told we had to park the car at the border and send someone on to get insurance before we could continue (which would likely have taken at least 12 hours). After an hours wait, they relented, no 'alternative solution' required, it was all just a power trip. But it is best to have it beforehand if you can, will save time and stress.

Also, we entered Mali from Mauritania via Fassala-Léré. In Léré, we were told by gendarmes and douanes to continue to Timbuktu, where we were going, and do all formailites there. En route, we were hassled by douanes in Tonka who said we should have got a laissez passer in Léré and would have to return to get it. In the end they kindly let us pay the fee for the laissez passer, and they let us continue. In Timbuktu, the same story - you can only get it at the point of entry, but we could pay the fee and continue. We had no further problems. BUT, without the laissez passer you can't get inusrance, which is a hassle, and hence the reason we arrived at the Niger border without. We heard of two other cars with the same problem of no laissez passer in Léré, so perhaps there it is a scam.

In any case - in Mali get your laissez passer at the point of entry, and get your insurance before going into Niger.

cheers
M.