In memory of my rebbi, Hagaon Harav Ahron ben Hagaon Harav Moshe Halevi Soloveichik zt"l on the 4th day of Sukkos, his eighth yahrtzeit.
Several points in no particular order:
1) Possible nafka mina based on this teshuva: If I know that someone intends to marry someone who is assur to him (or her), even if the issur is not known except to me, must I inform him? (Mamzer shelo noda may be an exception, but that’s midin mamzer shelo yada, not this more general rule discussed here). Probably I must inform the "kosher" person (whether it's a kohen about to marry a zonah or whatever else it may be), but if it's not a known psul, do I have to tell the local beis din? Not sure.
2) Rav Ashi – does he hold mesayei’a is chayav? In the context of this NB, when you leave the shatnez on, you’re mesayei'a by not contorting yourself into a position to remove it. When you remove it, you’re not chayav for the the time it takes to remove it, because in your action of removing it we see you do not want to be wearing shatnez, so you’re not mesayei’a. Why would I say a strange svara like this? This might be better for a Makkos post. Makos 21b (Noda Biyehuda calls it Machlokes Rav Beibo and Rav Ashi – but it is a machlokes Rav Acha brei D'Rav Ikka and Rav Ashi in response to a question posed by Rav Beibo.
My feeling - I don't think I can go through the sugya here, but does the din of mesayei'a apply only to cutting off peyos & beard, or does it apply to all aveiros? I felt, based on Tosfos, Shabbos 93b s.v. Amar Rav Zevid, that mesayaie'a only applies to peyos and beard. Then I saw my rebbi zt"l (whose yahrzeit is today) in his sefer Od Yisrael Yosef Bni Chai (siman 34) discusses the machlokes Shach and Taz in Yoreh Deah 198 about cutting nails for a woman who needs to go to mikva on shabbos or yom tov, mentions the whole Taz (se'if 21) and Shach (in Nekudos Hakesef) there. As Taz holds mesayei'a applies to other aveiros, like Shabbos. Rav Ahron strongly agreed with the Shach, that mesayei'a is only chayav for peyos. And he said the machlokes Shach & Taz is a machlokes Rambam & Raavad.
Rav Ashi is the one who says mesayei'a is chayav even as a nikaf; Rav Ashi in Shabbos 93b gives a proof that mesayei'a ein bo mamash, and Rav Ashi said if you leave the shatnez garment on you're chatav multiple times. In general Rav Ashi holds mesayei'a ein bo mamash except for peyos. Then why for shatnez are you chayav? So it's probably NOT a din in mesayei'a. Maybe the drasha (in Makkos 21 and 4b) "sadcha kilayim lo" applies to wearing shatnez also. But we get back to the question that it's lav she'ein bo maaseh, so why would you be chayav for not removing the shatnez garment? Maybe because in this case Rav Ashi holds mesayei'a is chayav. This is not compelling, but I don't know how else to explain it.
3) "David (i.e., Tehillim) for this matter will strengthen in his heart the love of his Creator." NB seems to hold like Baal Hatanya and not Nefesh Hachaim about torah lishma and tehillim. Baal Hatanya holds Torah Lishma is dveikus, love of Hashem & Torah, and saying Tehillim can lead to this. Rav Chaim of Volozhin in Nefesh Hachaim says Torah Lishma is studying Torah because you like it, for its own sake. Nefesh Hachaim brings a Rosh in Nedarim to support his opinion.
4) "we do not find isur mosif unless another prohibition is added or an obligation" – this is machlokes nosei keilim See Bach, Shach and Taz by basar bchalav (Siman 87) would the issur of basar b'chalav be added onto a piece of treif meat – chaticha naasis neveilah wouldn't apply to mechaber on a piece of treif meat but would apply to basar bechalav. Not sure how that halacha fits with NB.
5) "see Chagiga 9b that an adulterer's sin continues because he cannot rectify it."
Chagiga 9b says arayos is me’uvos lo yuchal liskon because you can never make her muteres l’baalah. It might be considered an isur moshech (a continuous sin) – Rogatchover in Tzofnas Panei’ach, Pinchas (Bamidbar 13, pp. 261-3) says that murder and being boel a non-Jewish woman are isur moshech, that continues on. Could be that arayos of eishes ish is also moshech, it continues, because there is a permanent effect on the woman – she is forbidden to her husband – because of the aveira. (In regard to murder, think of the passuk kol dmei achicha tzoakim when Kayin killed Hevel, Hashem said the voice of the bloods of your brother cry out - bloods for his blood and the blood of his descendants who will not be born now that he was killed.)
6) Is a sin of arayos, like eishes ish, considered bein adam l'chaveiro or bein adam l'makom? NB says eishes ish has two parts, the bein adam lamakom, and bein adam lachaveiro part in that he makes the woman forbidden to her husband (it's a sin against the husband).
Rambam seems to hold arayos is bein adam lamakom. See Hil. Teshuva 2:9 - "Teshuva and Yom Kippur only atone for aveiros bein adam lamakom like someone ate something forbidden or had a forbidden relationship (ba'al b'ilah asurah)." This is probably incest arayos, but Eishes Ish probably also has a bein adam lachaveiro aspect.
Gra in Aderes Eliyahu, Haazinu (Devarim 32:5), says there is a third category of sins - bein adam l'atzmo, and that arayos are part of this group.