Alternative proposal: Have the corsair spawn a barbarian corsair (maybe two) when it plunders a trade route. That way you can get the ability to harass the opponent with barbarians without having to sacrifice your own units to do so.
Good idea.

In the end, would you be more in favor of removing the capability for the Chinese UM2 to freely enter rival territory by adopting the method suggested by pineappledan (increased influence gain in non-ally CS territory) or in favor of removing the malus from the Corsair ? I think one of the two must be changed.
Idea : Did you look at the Greek UA ? I think one of the hidden bonus of it is that all Greek units can enter CS territory without being considered tresspassing.
They weren't laborers, so allowing them to build improvements wouldn't be accurate.
Maybe, but like for the Chasqui (I have bad souvenirs about this unit now... your responses weren't that friendly...), I think giving a worker action could make the unit more unique in a certain way : for the Chasqui, I still think the ability to quickly build roads would be a good thing and, as you said, not very difficult to code (what is your opinion about this, @adan_eslavo, @Blue Ghost ?) ; for the Sofa, giving them the ability to only repair improvements in order to make a conquered city quickly up and running or to repair fortifications wouldn't be the most accurate thing, but, once again, it would make the unit the "always busy" type.
The Farari (mandekalu cavalry), being nobility, were expected to finance and maintain their own weapons and armor. Sofa were purchased and outfitted by the state. I don't know how you reflect this difference in-game, but I don't think simply reducing maintenance would do the trick. If anything, their maintenance should be GREATER than your cavalry.
I agree with you on that one. I was simply some idea I had, but I knew it was in direct contradiction with historical facts...
The two units would play extremely differently. Since your cavalry don't have penalties vs cities they form the melee core, while the sofas accompany the GG and siege weapons.
I agree with you on this one.
Qadi (Courthouse)
I like that one better : it reflects how Songhai became a center of Islamic knowledge in Africa (one of the reasons I wanted a University replacement some time ago) and has an original interaction with religion that can be even more beneficial when you haven't founded your own (something we could only find in the vanilla Indonesia UB).