Bringing his fiancée to his father-in-law’s house for marriage

Ramban in Kiddushin, page 10, regarding the issue of marriage on a shekel, wrote that marriage with a prohibition is not permissible.

“What is a Canaanite slave whose marriage is not permissible because she is eaten with a gift, etc.” I asked him, “Whose blood is innocent marriage with a prohibition? And since it is prohibited, it is a complete prohibition, so how can it be permissible?” Perhaps it is possible to explain that marriage with a prohibition does not imply ownership.

As for the question itself, the Ritva asks this on page 10; a and explains that marriage for the purpose of marriage is permissible, even in the house of one’s father-in-law.

As for the Ramban, she brought in a prohibition for the same reason that she does not buy: because there is no kiddush in a Canaanite slave girl (as in a gentile). It cannot be learned from this that bringing in a prohibition does not buy, and certainly not bringing in something that is forbidden by the rabbis.
Regarding the Ritva, his words are a wonder to me. As I explained, there is no problem here. Perhaps he understood that it is completely permissible to come to his father-in-law's house, but I do not understand why it should be understood that way.
As I rewrote the words, I remembered the well-known act of Dr. Chaim Brisker (there is no such addition), and I said to myself that surely there is no such Ritva. And indeed, my understanding of the Ritva is there and I saw that he did not write this:

And our Rabbi the Gaon Alpas, may God be pleased with him, explained that this is a case of entering without a wedding, and that is similar to entering after a marriage of money or a deed of marriage that she made after she had consecrated and did not enter the wedding, buying her as a complete wife for her heir and defilement for her, namely, we say that she is a wife for her husband from where, and we used to say that she is a widow from the field of Ephron, meaning that she is a widow who takes and her husband, namely, entering and entering with jealousy, and we do not see that we do not find everywhere that entering after a marriage of money without a wedding, that she is a buyer, then entering is a forbidden marriage and bringing it to his fiancée in his father-in-law's house is a sin, and it is most proven in the Yevamot of the marriage of a deba, that without a wedding, a marriage of marriage without a wedding is adultery, except that it should be said that in a simple marriage, but in a marriage for the sake of marriage after a marriage of marriage, then he does Marriage, so wrote the Maimonides.

What he writes is that if a woman comes for the sake of marriage, she is committing a marriage and is not fornication. He does not write that there is no prohibition in this (Darbanan, coming to one's wife in one's father-in-law's house. There is no prohibition in fornication because this is truly not fornication).

The Rabbi understands that when the Ritva says, “They came to him for the sake of marriage,” this is only referring to the gemen in the Yevamot and not to the words of the Ramban.
A. I do not understand if this bride is performing a marriage, why did she get a shilk? What is wrong with this bride?
B. The Ramban instead says, “But I do not know of a bride without a chuppah that she would buy for inheritance and other types of marriage, because a bride is forbidden and is like adultery, and she will buy it, and the N is proven, because Achin, page 29, does not buy it, but the Ram, his disciple, says that a bride who comes to his fiancée for the sake of marriage is her chuppah. You say that a bride buys for the sake of marriage, and a chuppah buys for the sake of marriage, and a bride who comes to his fiancée in his father-in-law’s house for the sake of marriage does not buy it, and they are wrong in their words.” Here it is clear that the one who marries his fiancée in his father-in-law's house for the purpose of marriage is not guilty, as I explained in the Ritva. And in the Ritva, it seems to me that this is the explanation because he wrote, "And so the Ramban" and this is the Ramban that the Ramban cites.
C. It seems to me that the aforementioned Ramban sees that the prohibition does not apply to a buyer regardless of the nature of the property.

A. As stated, there is a rabbinical prohibition. We are talking about rebellious beatings, not slander. They also simply forbade the act of consecrating a bride (because of someone who threw his wife under the tree), but I don't think anyone would disagree that if he did so, she would have brought a buyer.
B. Here too, the precision is not necessary. It is possible that the one who came for the sake of marriage is ill and a buyer.
C. His intention there is not clear to me. Perhaps the reason there is a prohibition is because it is clear that his intention is not to buy but to fornicate. And it is also possible that we are really talking about prohibitions that involve fornication and not any prohibition (like consecrating a regular bride, which I don't know of anyone who says they don't buy because there is a rabbinical prohibition). Or maybe he is repeating that this is as invalid as a mitzvah that comes with a sin? It seems narrow to me (first of all, because it is not a mitzvah, and more). In short, I see no reason why the prohibition on consecrating a bride would prevent the occurrence of a consecration or marriage.

My initial understanding was that the Sages forbade a groom from visiting his fiancée before the marriage stage (because there are some kind of hirits in this). Visiting his fiancée at his father-in-law's house is just a common example of visiting for reasons other than marriage (because an ordinary person does not intend to do so when visiting his fiancée, but the surrounding circumstances are a literal manifestation of his general intention - if they held a wedding and a banquet, then this is a sign that the groom intends for marriage, and if they did not, and the fiancée is still at his father-in-law's house, then this is a sign that he does not intend for marriage). From this I concluded that even if the visit was at his father-in-law's house, but for the purpose of marriage, then there is no problem here. For example, nowadays it is common for the bride and groom to live in the parents' house of one of the spouses. In such a case, there seems to be no problem.

I went halfway with you. The prohibition indicates that there is no intention here. And perhaps this was the reason for the prohibition itself. But since it is prohibited, it is prohibited. But it does not necessarily have a connection to the concept of marriage. In other words, if one intends to marry, it will be subject to the strictness of the prohibition.
As for the explanation, it seems to me that the ”father-in-law's house” is not a geographical place, but rather as long as the woman is in the possession of her father-in-law and is not married to him. Therefore, when a husband lives in her parents' house, it is also his house and not his father-in-law's house.