I hold that validly baptized Christians, who believe explicitly in the Trinity and Incarnation, confess Jesus Christ as Lord and Savior, and err in good faith, are real Christians. Msgr. Fenton says such can be saved if they are in good faith.
You "hold" error. No, they are not REAL Christians. Here you go again subjectivizing faith. Those "Christians" can believe anything they want, but if they don't believe it on the basis of the proper rule of faith, they do not have the proper formal motive of faith. St. Thomas clearly teaches this, that even though such people materially happen to believe some true doctrine, if they do not hold it with the correct rule of faith, then they do not have the supernatural virtue of faith. Whether they are in good faith or bad faith, if they lack a supernatural faith based upon the rule of faith established by God, then their rule of faith ultimately reduces to believing what they want to believe. If they are in good faith, then God will undoubtedly bring them to the true faith.
If these people are able to be saved, then they are in the Church, since there's no salvation outside the Church. Therefore, you too hold the false Vatican II ecclesiology where the Church subsists in the Catholic core but then extends outside of it.
FORMAL heresy has nothing to do with "sincerity" and "good faith". Formal heresy refers to whether or not someone believes what he believes with the correct formal motive of faith that rests upon the proper rule of faith. Material heresy deals with ignorance or a mistake in understanding WHAT has been taught and must be believed. Formal heresy refers to WHY it is believed. That is why it's said that one who rejects a single dogma rejects all of them because it's an implicit repudiation of the rule of faith on which all dogma rests. Protestants are all by definition formal heretics, regardless of how "sincere" they are.
It would be one thing if you referred to these people as Christians in a loose sense, but calling them REAL Christians? That's absurd. You've been reading too much Valtorta.