Just thinking out aloud here...
Looking at the scale, Nkout to Mbalam is ~ 120km line of sight.
Actual path of a winding railway undoubtedly longer.
(The reference to 30km above in Ecomatin article is grossly incorrect - perhaps they meant 130km.)
I wonder if it's been contemplated to stop building the rail line at Nkout?
Then in the future if/when the Mbalam & Nabeba/Babondo/Avima litigation is sorted, extend the rail to Mbalam and then the spur line into Rep of Congo.
Would minimise up-front capex.
That's what I would do, anyway. ;-)