If Shaba is successful then the Lunda are going to be about as mercenary as they could be in selecting a patron. The conflict had more in common with Biafra then an ideological conflict. It was an ethnic conflict with ideological overlays. The FNLC got its start in Angola fighting for the Portugese, not fighting for the MPLA, and they were only backed by the MPLA for an attempt on Shaba after the FAZ was deployed against the MPLA with the usual disastrous consequences and war crimes that seem to have been a hall mark of that military. The intervention lasted less the two months and spurred the MPLA into supporting the FNLC's stated goals of reclaiming their homeland.
Given this history, you can bet that if the FAZ falls apart faster, or the Moroccans and the Safari club are just a little slowed off the mark then we might see a repeat of the Katanga wars. Or the US could have reached for the Portuguese government to work as an intermediary with the FNLC while the FAZ, and all of Zaire, went up in a civil war. Given Mobutu called in the Belgians for SHABA II it should be very clear just how fragile his position was at this time. If Zaire falls apart into a civil war then i would bet that the west would induce the Lunda and FNLC to get a bit expansionist, as both Angola and Zambia had regions where they were the majority ethnic group. Given that China backed Zambia, and the USSR backed Angola through its Cuban proxies the FNLC could very well have ended up with a reputation as an ethnic army instead of the leftist cloak they wore.
TL

R No the FNLC could not have overthrown Mobutu but they could have touched off a civil war which he could have lost. Zaire would not end up a true 'communist state' without a lot of outside assistance for the communist faction of the civil war and the FNLC, as representatives of the Lunda, would have been one of the Wests pry bars to keep the communists from having safe basing during the civil war. They might have been able to parlay that into a successful succession, but i doubt it.