I don't see how Redemptoris missio deals with infants and the mentally-challenged so I am unclear why this is an issue for you.As I hope will be clear by now, the case of infants and the mentally handicapped demonstrates that there are at least two classes of people who are unable to make a Protestant profession of faith.
Unless she is going to say (and I hope that she would not, most sincerely) that they are all going to hell, then there are at least two classes of people where she and I agree that God saves them - whether some of them or all of them makes no difference at this point - through Christ, apart from the normal means (we of course differ on the "normal means," but that is not relevant at the moment).
That being the case, it hardly seems objectionable to suppose that there are people of a third class whom God might also save through Christ and apart from the normal means: namely, those who through no fault of their own have never had the opportunity even to hear of Jesus Christ.
This is what the Pope asserts in brief in RM: that there are such people. But (as he makes perfectly clear throughout the (rather long) encyclical) this in no way changes the fact that the Church must proclaim salvation through Christ, and offer it through the ordinary means. The obvious implication is that it would be a fool's errand to place one's hope for salvation in something apart from the ordinary means that God has provided.