A return of the monarchy would only be possible in Salazar's time if he desperately needed the support of the monarchists to maintain his regime. He deliberately kept things vague regarding a possible restauration of the monarchy in order not to alienate the monarchists, but Salazar himself was not a monarchist (and I remember reading somewhere on a academic paper on the net that Salazar even helped push away his old party from the days of the First Republic (the Catholic Centre) from Monarchism to Republicanism.
The keeping of Cape Verde and São Tomé and Príncipe would be possible with a democratic transition less influenced by the far-left, since the population there was generally supportive of Portugal. The same applies to East-Timor.
Angola while was a military front that was being won, and assuming Portugal kept its forces there for a while more, would sooner or later revert to some form of independence, either as a sort of Commonwealth or full independence, but close to Portugal.
There would be a need for full franchise and besides some parts of the population pushing for independence, some currents of thought in Portugal would have also pushed for independence for Angola given Angola's population and the concept of forming new countries with Portuguese cultural elements embedded in them.
Among the monarchists there is a minor faction associated with the People's Monarchist Party that considers that the House of Barreto (the Dukes of Loulé) are the legitimate claimants.
I hope this helps.