Mobutu using threat of the USSR? The UN was working in parallel with the USSR, in delivering weapons to the Congo, that were used to attack Katanganese, to force surrender of its land to the Communists of the Congo.
Tshombe was only ever president over Katanga not the whole Congo.
Katanga was utterly forced to entirely surrender to the communist Congo, by the third and most devastating allout attack from the UN, in December of 1962.
One thing you could say is the Congo could have had retrobutal based grudges against Katanga, for the atrocities done by the Europeans, throughout the Congo.
On the contrary, Katanga faced attacks by the rest of the Congo, before Europeans were ever in the Congo.