Why is the krei/ketiv of yishgalena/yishkavena not a problem of reading not from the ktav?

A famous krei and ketiv pair in parshat Ki Savo is in Devarim 28:30:

ל אִשָּׁה תְאָרֵשׂ, וְאִישׁ אַחֵר ישגלנה (יִשְׁכָּבֶנָּה)--בַּיִת תִּבְנֶה, וְלֹא-תֵשֵׁב בּוֹ; כֶּרֶם תִּטַּע, וְלֹא תְחַלְּלֶנּוּ.

30 Thou shalt betroth a wife, and another man shall lie with her; thou shalt build a house, and thou shalt not dwell therein; thou shalt plant a vineyard, and shalt not use the fruit thereof.

As I discussed in the past, a convincing reason for this krei and ketiv is that over the centuries, language changed, and what was once an acceptable word became unacceptably crude. Just as many English curse words nowadays.

This krei and ketiv is mentioned in Megillah 25b, where it is clear that the problem is that it is inappropriate:

ת"ר כל המקראות הכתובין בתורה לגנאי קורין אותן לשבח כגון (דברים כח) ישגלנה ישכבנה (דברים כח) בעפולים בטחורים (מלכים ב ו) חריונים דביונים (מלכים ב יח) לאכול את חוריהם ולשתות את מימי שיניהם לאכול את צואתם ולשתות את מימי רגליהם

And the Rambam discusses this in hilchot sefer Torah, that it should not chas veshalom be written in accordance with the krei, for that would be an invalid sefer Torah:

רמבם ספר אהבה הלכות תפילין ומזוזה וספר תורה פרק ז הלכה יא
וכל הדברים האלו למצוה מן המובחר ואם שינה לא פסל. אבל אם כתב המלא חסר או החסר מלא. או שכתב מלה שהיא קרי וכתב כקריאתה כגון שכתב ישכבנה במקום ישגלנה ובטחורים במקום ובעפולים וכיוצא בהן. או שכתב פרשה פתוחה סתומה או סתומה פתוחה. או שכתב השירה כשאר הכתב. או שכתב פרשה אחת כשירה הרי זה פסול ואין בו קדושת ספר תורה כלל אלא כחומש מן החומשין שמלמדין בה התינוקות:

Thus, the ketiv certainly has importance in terms of being the required Biblical text.

Minchas Shai discusses this krei and ketiv, which is mentioned in the gemara and masechet Soferim. And then he cites the Rashba, who wrote in response to the question that since it is forbidden to read in the Torah even a single letter not from the text, how can the baal koreh read yishkavena when the text is yishgalena. And so too, every word which has a krei and ketiv, since all of them are written in the Torah according to the masoret (consonantal text) and not according to the reading (/vocalization) -- basically, every krei and ketiv. The answer is that this is a halacha leMoshe miSinai, as is written in perek Ain Bein HaMudar, Nedarim 37b:

אמר רבי יצחק מקרא סופרים ועיטור סופרים וקריין ולא כתיבן וכתיבן ולא קריין הלכה למשה מסיני

The ones which one reads, even though there is no word there at all, are as follows:

קריין ולא כתיבן (שמואל ב ח) פרת דבלכתו (שמואל ב טז) איש דכאשר ישאל איש בדבר האלהים (ירמיהו לא) באים דנבנתה (ירמיהו נ) לה דפליטה (רות ב) את דהגד הוגד (רות ג) אלי דהגורן (רות ג) אלי דהשעורים הלין קריין ולא כתבן

while the ones which are present, though one does not read, are:

וכתבן ולא קריין (מלכים ב ה) נא דיסלח (דברים ו) זאת דהמצוה (ירמיהו נא) ידרוך דהדורך (יחזקאל מח) חמש דפאת נגב אם (רות ג) דכי גואל הלין כתבן ולא קריין

Presumably this means an old tradition, rather than all the way to Sinai, since most of the examples are not Pentateuchal. It does not mention this particular sort of krei and ketiv in which one reads an entirely different word for the written word, but perhaps one can derive this by simple combination of the two sorts.

He continues that there was an incident with one who read as it was written {I suppose yishgalena} before the Gedolei Hador, and they admonished him that he should read according to the masoret {here, this means according to the traditional way it should be read} and he did not desire to do so; and they placed him under the ban and removed him from the teva.

It takes a specific type of personality for this, I suppose. I recall my brother-in-law, when laining in the YU Bet Midrash, refusing to modify his pronunciation of patach ganuv mapik heh, where he insisted on laining it ha (with an extended patach) rather than ah; despite being corrected multiple times. For he felt he was correct, following a tradition from his father and grandfather.

In the case in question, about krei and ketiv, perhaps this was some fellow with Karaitic leanings. Though I see that Aharon ben Yosef the Karaite is perfectly happy with this krei and ketiv. Perhaps the "problem" was indeed this injunction against not reading anything not from the text.

In Igros Moshe, Orach Chaim 1:32, Rav Moshe Feinstein makes it clear that he does considerthis reading from the text. That is,

ומסתבר רהוא הדין בקרי וכתיב כגון על תיבת

ישגלנה נמי יצטרך להוציא אהרת משום שבהלכה

נאמר שתיבת ישגלנה בכאן יקרא ישכבנה ולא שיקרא

תיבת ישכבנה בע"פ.

That is, if wax drips and obscures the word, it is logical that you need to bring out another one, for the halacha is that looking at the word ישגלנה, you should read ישכבנה, but not by reading it baal peh.

A related question is just how crude the word is. Rambam, in Moreh Nevuchim 3:8, tries to declare lashon kodesh holy in that it has no crude words, and so he explains the meaning of ישגלנה in a non-crude manner.