Md,
:Gaon, as a side point, that’s not what Rav Chaim Brisker meant. Otherwise, Tzedukim who were never exposed to real Yiddishkeit would not be considered tinokos she’nishbu.”
Sorry, but That is exactly his chidush in the very Rambams shitah where the wording of the Rambam in pirush hamishnayos on perek Chelek indicates so. The Rambam states after establishing the Yud Gimel Ikrim
Mוכאשר יאמין האדם אלה היסודות כלם, ונתבררה אמונתו בה”
“הוא נכנס בכלל ישראל
We see clearly from the very words of הוא נכנס that it’s a conditional matter of what makes and defines a Jew as a Jew. Ignorance is only applicable for all other mitzvos, but not for these. That is how Rav Chaim explains it. And that is the reason why the Rambam has 13 Ikrim and not 3 of 4 as others have, being that the above is applicable to all 13 .
However, you did raise a good point about the Halcha of Tinuk shenishba. On that, I heard that the Brisker Rav pointed out that in the old accurate prints of the Rambam at the Halcha of Tinuk shenishbu, the Rambam adds. I will quote (and it is printed in the updated rambams)
פ”ג מהל’ ממרים
ג2 אֲבָל בְּנֵי אוֹתָן הַטּוֹעִים וּבְנֵי בְּנֵיהֶם, שֶׁהִדִּיחוּ אוֹתָם אֲבוֹתָם וְנוֹלְדוּ בְּמִינוּת וְגִדְּלוּ אוֹתָם עָלָיו – הֲרֵי הֵן כְּתִינוֹק שֶׁנִּשְׁבָּה לְבֵין הַגּוֹיִים וְגִדְּלוּהוּ הַגּוֹיִים עַל דָּתָם, שֶׁהוּא אָנוּס; וְאַף עַל פִּי שֶׁשָּׁמַע אַחַר כָּךְ שֶׁהָיָה יְהוּדִי, וְרָאָה הַיְּהוּדִים וְדָתָם, הֲרֵי הוּא כְּאָנוּס, שֶׁהֲרֵי גִּדְּלוּהוּ עַל טָעוּתָם – כָּךְ אֵלּוּ הָאוֹחֲזִים בְּדַרְכֵי אֲבוֹתֵיהֶם שֶׁתָּעוּ. לְפִיכָךְ רָאוּי לְהַחֲזִירָן בִּתְשׁוּבָה, וְלִמְשֹׁךְ אוֹתָן בְּדַרְכֵי שָׁלוֹם עַד שֶׁיַּחְזְרוּ לְאֵיתַן הַתּוֹרָה, וְלֹא יְמַהֵר אָדָם לְהָרְגָן.
The last sentence “ולא ימהר אדם להרגן” was omitted from the semi old prints (for obvious reasons) which basically means that מעיקר הדין בעצם he has a status of a MIN ומרידין ולא מעליו however being that he is a תינוק שנשבה ראוי תחלה “להחזירו בתשובה”
Note, the Chazon Ish in YD however disagrees.