Can board a plane to Cameroon without a Cameroonian visa?

According to Timatic, the system used by most airlines in determining if you meet the requirements for travel to a specific country, a US citizen CAN travel to Cameroon without a visa in their passport under the following conditions :

Passengers with a confirmation letter that a visa has been approved can obtain a visa on arrival. The letter has to be issued by the competent authorities of Cameroon.

Presuming the "negotiations" you refer to include a letter that meets these requirements, then yes, you will be allowed to travel.

The actual visa policy of a country is not always exactly the same as what is stated in the relevant airline manuals or on wikipedia. E.g. even though the manual says that you cannot get a visa on arrival, in reality this may be possible.

HOWEVER: people at the checkin counter will only trust their manual. The only exception would be that an airline representative tells them that a visa on arrival is possible. This sometimes happens when flights are flown by national carriers (e.g. Air Cameroon). Such a scenario is rather unlikely if you fly Air France and have connecting flights. It is not a risk I would be willing to take unless I were very certain that this works for my particular connections and my particular airlines.