We can resolve the question the Maharil Diskin asked on Rashi in a different fashion as well:
It is clear from a number of places that the primary לאו of גזילה is not because of the very act of גזילה but rather the foundation of the איסור is a prohibition against acquiring an object in the context of קנייני גזילה.
The proof is from what we asked in Temurah [6a] in the sugya of אי עביד לא מהני.
והרי גזל דרחמנא אמר (ויקרא יט, יג) לא תגזול ותנן הגוזל עצים ועשאן כלים צמר ועשאן בגדים משלם כשעת הגזילה
The Gemara objects: But there is the case of robbery, with regard to which the Merciful One states: “You shall not oppress you neighbor, and you shall not rob him” (Leviticus 19:13), and we learned in a mishna (Bava Kamma 93b): One who robs another of wood and fashions it into vessels, or one who robs another of wool and fashions it into garments, pays the victim according to the value of the goods at the time of the robbery, but he need not return the vessels or garments, since by changing the stolen items he acquired them.
In other words, we learn from this that that the thief acquires with a שינוי and we don't say "לא מהני" - that what he did sinfully is not effective. Asked the Yad Hamelech: Even if we say "לא מהני" the איסור won't be rectified and the Maharit [and other Acharonim] taught the rule that we only say אי עביד לא מהני if the איסור will be rectified. It appears from this that the איסור גזילה is not on the very act of stealing but rather on the קנייני גזילה that he has on the stolen object, and because of this the Gemara asks correctly that we should say "לא מהני" and the קנין shouldn't take effect. This way, the איסור of גזילה will be rectified. That is what the Gemara means when it asks why the person is קונה בשינוי and that we should say לא מהני and there is no קנין.
With this insight in hand we can understand the sugya in Bava Metzia [10b] that teaches according to one opinion that wherever the שליח is not obligated [a בר חיובא] we say יש שליח לדבר עבירה - he is a valid agent and his act relates back to the sender. Asks the Gemara:
אלא מעתה האומר לאשה ועבד צאו גנבו לי דלאו בני חיובא נינהו הכי נמי דמיחייב שולחן
The Gemara asks: If that is so, then in a case of one who says to a woman or a slave: Go out and steal for me, is the one who sent them indeed liable, since they are not subject to liability? [Married women and slaves have no property of their own from which one could collect payment].
אמרת אשה ועבד בני חיובא נינהו והשתא מיהא לית להו לשלומי
The Gemara answers: You can say in response: A woman and a slave are not comparable to a courtyard, as they are subject to liability if they steal; and only now, in any event, they do not have the means to pay.
Asked Rebbe Akiva Eiger, what is the hava amina of the מקשן [questioner]? For even if we say that the reason she doesn't pay is NOT because she lacks the means but because she is exempt, nevertheless she is definitely included in the איסור of גזילה and is thus considered a בת חיוב in the איסור גזילה and cannot be a שליח?!
Based on the foregoing we can explain based on the principle of the Nesivos, that if the גזילה was done in such a way to incur קלב"מ then there is no קנין בשינוי. Since it is not included in the obligation of והשיב [because of קלב"מ] and there is no obligation to pay, there is also no קנין גזילה and thus he can't be קונה with a שינוי. According to this we can say that since in the hava amina we assumed
Based on the foregoing we can explain based on the principle of the Nesivos, that if the גזילה was done in such a way to incur קלב"מ then there is no קנין בשינוי. Since it is not included in the obligation of והשיב [because of קלב"מ] and there is no obligation to pay, there is also no קנין גזילה and thus he can't be קונה with a שינוי. According to this we can say that since in the hava amina we assumed
that a woman is exempted from payment and not just that she doesn't have what to pay, she will then have no קנייני גזילה [no "והשיב" - no קנייני גזילה] and the questioner correctly assumed that a woman is not included in the לאו of לא תגזול. Because as we explained, the לאו relates to קנייני גזילה and not to the גזילה itself and since she has no קנייני גזילה in the object, she is also not included in the איסור גזילה.
Now we can answer the question of the Maharil Diskin on Rashi - He asked why there is a בקנין שינוי when a בן נח steals when according to the Nesivos if קלב"מ applies and there is no והשיב, there is no קנין שינוי. And for a בן נח where קלב"מ always applies and there is no דין of והשיב, the קנין בשינוי shouldn't apply. But based on what we explained, it emerges that if we say that there is no קנין בשינוי then there is no איסור גזילה at all because the whole לאו is not to make קנייני גזילה in an object, and we are thus compelled to say that a בן נח IS קונה with קנייני גזילה [because we know that he is included in איסורי גזילה as the Gemara learns in Sanhedrin 56b] and thus is also קונה בשינוי.
To summarize: The question was that a בן נח shouldn't be קונה בשינוי according to the rule of the Nesivos that if there is no והשיב, there is also no קנין. The answer is that the only way that an איסור גזילה can apply is if there are קנייני גזילה and since a בן נח has an איסור גזילה, he must also have קנייני גזילה. This undermines the rule of the Nesivos and we must say against his view that even when there is no והשיב, there are קנייני גזילה.
[עפ"י תורת הגאון רבי חיים שמואלביץ זצ"ל]
Now we can answer the question of the Maharil Diskin on Rashi - He asked why there is a בקנין שינוי when a בן נח steals when according to the Nesivos if קלב"מ applies and there is no והשיב, there is no קנין שינוי. And for a בן נח where קלב"מ always applies and there is no דין of והשיב, the קנין בשינוי shouldn't apply. But based on what we explained, it emerges that if we say that there is no קנין בשינוי then there is no איסור גזילה at all because the whole לאו is not to make קנייני גזילה in an object, and we are thus compelled to say that a בן נח IS קונה with קנייני גזילה [because we know that he is included in איסורי גזילה as the Gemara learns in Sanhedrin 56b] and thus is also קונה בשינוי.
To summarize: The question was that a בן נח shouldn't be קונה בשינוי according to the rule of the Nesivos that if there is no והשיב, there is also no קנין. The answer is that the only way that an איסור גזילה can apply is if there are קנייני גזילה and since a בן נח has an איסור גזילה, he must also have קנייני גזילה. This undermines the rule of the Nesivos and we must say against his view that even when there is no והשיב, there are קנייני גזילה.
[עפ"י תורת הגאון רבי חיים שמואלביץ זצ"ל]
