I think it cold happen. Gaddafi would need also to avoid bombing Pan Am and a few other things. If he then would be the main financial backer of the Lake Chad program, then it could be completed. It is roughly the same size as the Colorado to central valley of California project. The Chad project requires 3 dams. One at where some flow of the Congo is divert to the Ubangi. You then build a diversion canal down hill to the Ubangi. You need a dam at the Ubangi. You then need a second dam right below this diversion. You then build the channels to lake Chad. The final dam is the Exit to Lake Chad. You need to get full map of Lake Chad to understand. There are two basin of Lake Chad. The one that has the lake is the smaller basin at a higher elevation. You need the third dam to control the level of Lake Chad.
Huge problem is cutting across national borders like that. Gaddafi would need to sell himself as the ultimate pan-Africanist (which later on, he kinda did), plus have friendly pan-Africanists ruling in the relevent countries.
Environmental impacts.
Increase rainfall in western Sahara.
Refilling Lake Chad.
Large Salt basin created in Northern Chad.
What do you mean salt basin in Northern Chad? Lake Chad is freshwater.
As to the diversion of water from the Ubangi, I don't know that it would be a small amount; and officials of the CAR, already a few years ago, voiced concern not so much about the Congo but about the Ubangi. The Transaqua project would reduce the energy potential of dams in the CAR, might make navigation more difficult, and would, most importantly, reduce the fish catch. A reduction of available food in Africa is no small concern.
I don't buy that explanation. It would reduce the energy potential of the dams on the Congo, but not by a large margin (and energy isn't too much of a concern in Africa at the moment), and navigation, well, you still can't go further than the port of Bangui from the Congo, and the Congo is such a big river that I can't imagine taking some water from it would impede navigation on the Congo itself. Fishing, why would it significantly reduce the fish catch? In any case, it might spur more migration to cities in the Congo/CAR as a result of less fish catch.
Now wouldn't the majority of the rainfall end up in Sudan and the Horn of Africa? I suppose that would be good, since it would increase the amount of rain going to the Nile. Or otherwise you could easily present it as doing so, and hope to get the Egyptians, Sudanese, and Ethiopians on your side to help finance the project.
That makes no economic sense, and it may well be physically impossible.
Yeah, it would just vanish in the Sahara. A better solution is to flood the
Sabkhat Ghuzayyil plus the smaller chott between it and the port of Brega, connecting them to each other and to the sea with a canal. This would make two decent sized lakes which are freshwater lakes and thus can be used to irrigate the desert, stock fish in, and possibly for navigation almost 100 kilometers inland.