It bears pointing out that invincible ignorance doesn’t save in and of itself, it only excuses certain sins–one must still have supernatural faith, without which it is impossible to please God.
In regards to baptized Protestants, it depends whether or not they are heretics or schismatics–neither can be saved. However, one must properly define those terms. In fact, both require a spirit of obstinacy. St. Augustine explains:
“Those are by no means to be accounted heretics who do not defend their false and perverse opinions with pertinacious animosity, especially when their error is not the fruit of audacious presumption but has been communicated to them by seduced and lapsed parents, and when they are seeking the truth with cautious solicitude and ready to be corrected.”
If God willed it then Yes, of course it could happen. Does God will it? The testimony of Scripture seems to be in the affirmative, although it’s difficult to be sure that there was nothing in the context to remove the force of the famous, “[God] desires all men to be saved and to come to the knowledge of the truth.”. We can’t split God in pieces so that His will is opposed to His desire, so Yes, God does will it. Now all that remains is for humans to cooperate with God’s will.