Wanted to share a positive story for crossing at Diamma with visa obtained in Nouakchott! The Start of the Piste is now being tarred, but it's not tarred all the way. I skipped off at one point and rode on the old piste(for 500m) before getting back onto the prepared gravel road that was still un tarred.

Mauritania:
2000UM for the park entrance (receipt with 2000 printed on it)
500UM for the communal tax (hand written receipt)
Gendarme asked kindly for a "cadeau", but nothing further. The young Douane officer asked for 10€, refused to stamp. I asked to speak to the boss, who just waved me through, no issues. Police, no issues, very nice!

Bridge across the river, 4000CFA official receipt, hand written. No hustlers.

Senegal:
Police hassled for a few minutes for 10€, but gave up quite quickly
Douane guy was really nice, didn't ask for a bribe and even stamped my Carnet! No passevant required. (My friend who crossed just before me suspected that the more troublesome officer had just left on his arrival)

Douane and Gendarme stopped us on the way to St. Louis, checked the Carnet and a bit of paperwork. No issues, just a nice chat about Senegalese football players and a quick introduction to wolof
Policeman south of St Louis just waved us by: no "violations for wrongly indicating!"

All in all, a great day. Lady luck was on our side today?

Africa on a KTM690 Overland Ride | Cornwall to Cape Town