There is a major problem that is legitimacy.
Once you remove the colonial narrative, there are no grounds to establish any sort of colonial legitimacy.
Well, I'd argue that legitimacy is only of limited importance within a cold war context.
But anyway, if there is a plebiscite in some areas of Angola and Mozambique, and the plebiscite shows that a majority of the population is willing to stay a part of Portugal under the new federal model, then there is a new type of democratic legitimacy, which should be more than enough to win widespread western support for the federation project if that is in line with American/western economic interests.
And from a exploitation of resources by international conglomerates perspective Portugal would become the middle man. It always makes business sense to remove the middle man.
I don't really get what you mean "middle man" in this case. Portugal would be the government of these territories, a very pro-American, pro-western government which would surely provide terrific conditions for investment. It would also be the government that would provide a level of order and stability unmatched in Africa, as well as the government that would be keeping these regions and their resources firmly out of communist hands.
It would make no sense for the west to start funding guerrillas and botch that.
There is a reason why nobody was able to hold on to large colonies after WW2 for long. The model was obsolete.
I think it's important to avoid being too absolutist when discussing alternate history, but in this case the model would be replaced with something else.
Looking at the scenario, I don't see any reason why it should have to necessarily fail.