It is clear from his words that a hupat niddah does not effect marriage, and in the Gemara [Ketubot 56a] it is an inquiry: Rav Ashi inquired, if she entered the hupah and became a niddah, what is the law? If you say that the affection of the hupah effects acquisition—a hupah that is fit for intercourse, but a hupah that is not fit for intercourse, no; or perhaps it makes no difference? It remains unresolved [teiku], see there. Therefore, he ruled stringently, and according to this, the Rambam should have written that it is a doubt. And granted that regarding the prohibition there is no practical difference in this, nevertheless regarding monetary matters, such as to inherit from her, there would be a practical difference if he seized [property], for we do not extract it from his hand as in every legal doubt. And behold, the Rambam holds regarding every teiku in the Shas that seizure is effective, as he wrote in Choshen Mishpat in several places [and so wrote the author of Sefer Tekufo Kohen siman 8, and so wrote the Urim VeTumim there]. And more than this, if the husband is in possession of the dowry, the heirs of the wife do not extract it from his hand, and from his words it does not imply so, since he ruled that it is not marriage in a hupat niddah.
And it is possible to say that regarding inheritance there is no practical difference in this, for he certainly inherits from her according to what he wrote in siman 57, that if the father delivered her to the messengers of the husband, he inherits from her even if she is not fit for intercourse, as he wrote there in section 5. And all the more so in a hupat niddah, which is no worse than this, for behold, he brought her into his domain. And according to this, also regarding vows and that her death is by strangulation, her law is also as a married woman, as he wrote there in section 1, but it does not imply so from his wording. And the Maggid Mishneh wrote explicitly that he does not inherit from her, see there. And some say that the Rambam follows his reasoning in the first chapter of Hilkhot Nachalot, that in a doubt of divorce the husband does not inherit from her, and all the more so in a doubt of kiddushin [Sha'ar HaMelekh]. And there is no proof, for certainly even there seizure is effective, and from where do we know that seizure is not effective? And behold, many of the poskim disagreed with his primary ruling, as he wrote in siman 90, and all the more so that in any case, even according to the Rambam, seizure is effective; for it is obvious that if he seized, a doubt of divorce is no worse than "the house fell upon him and upon his wife," which will be explained there [and so wrote the Beit Shmuel there subsection 20, that as a matter of law seizure is effective, and the Chelkat Mechokek there subsection 14 compared this to "the house fell," see there].
מבואר מדבריו דחופת נדה אינו עושה נשואין, ובגמ' [כתובות נ"ו.] היא איבעיא, בעי רב אשי נכנסה לחופה ופירסה נדה מהו, את"ל חיבת חופה קונה – חופה דחזיא לביאה אבל חופה דלא חזיא לביאה לא, או דילמא לא שנא? תיקו, ע"ש, ולכן פסק לחומרא ולפ"ז הו"ל להרמב"ם לכתוב דהוה ספק, ונהי דלעניין איסורא אין נפק"מ בזה מ"מ לעניין ממון כמו ליורשה הוה נפק"מ אם תפס דאין מוציאין מידו כבכל ספיקא דדינא, והרי הרמב"ם ס"ל בכל תיקו שבש"ס דמהני תפיסה כמ"ש בחו"מ בכ"מ [וכ"כ בס' תקפו כהן סי' ח', וכ"כ האורים ותומים שם], ויותר מזה דאם הבעל מוחזק בהנדוניא אין מוציאין יורשי האשה מידו, ומדבריו לא משמע כן כיון שפסק דאינו נשואין בחופת נדה, ואפשר לומר דלעניין ירושה אין נפק"מ בזה דוודאי יורשה לפי מ"ש בסי' נ"ז דבמסר האב לשלוחי הבעל יורשה ואפילו אינה ראויה לביאה כמ"ש שם סעי' ה', וכ"ש בחופת נדה דלא גרע מזה שהרי כנסה לרשותו, ולפ"ז גם לעניין נדרים ושמיתתה בחנק דינה ג"כ כנשואה כמ"ש שם סעי' א', ולא משמע כן מלשונו, והמגיד משנה כתב מפורש דאינו יורשה ע"ש, וי"א דהרמב"ם הולך לשיטתו בפ"א מנחלות דבספק גירושין אין הבעל יורשה וכ"ש בספק קדושין [שעה"מ], ואין ראיה דבוודאי גם שם מהני תפיסה ומנלן דלא מהני תפיסה, והרי על עיקר דינו נחלקו הרבה מהפוסקים כמ"ש בסי' צ' וכ"ש דעכ"פ גם להרמב"ם מהני תפיסה, דפשיטא דבתפס לא גרע ספק גירושין מנפל הבית עליו ועל אשתו שיתבאר שם [וכ"כ הב"ש שם סק"כ דלדינא מהני תפיסה והחמ"ח שם סקי"ד דימה זה לנפל הבית, ע"ש].