A clarification first
You must differ between a Sahih book like Sahih Muslim and Sahih al-Bukhari and the Sunan books. While al-Bukhari and Muslim intended to only quote Sahih hadith (based on their conditions) in full chain in their books. The authors of the Sunan etc. mainly had a different intention, mostly they quoted ahadith that scholars used as an evidence so they had a lower level of sanity.
So the qualification saying that the ahadith from Sunan an-Nasa-i is Sahih was made by the hadith scholar(s) Darussalm rely on Hafiz Zubair 'Ali Za'i (see here) and is not indisputable.
The reasons for revelation of this verse which have been counted
In fact there are on the whole three different reasons of revelation that have been quoted concerning the story covered in the verses (66:1-5).
Imam al-Qurtobi mentioned them in his tafsir (see for example here):
'A'isha (Allah be pleased with her) narrated that Allah's Apostle (ﷺ) used to spend time with Zainab daughter of Jahsh and drank honey at her house. She ('A'isha further) said:
I and Hafsa agreed that one whom Allah's Apostle (ﷺ) would visit first should say: I notice that you have an odour of the Maghafir (gum of mimosa). He (the Holy Prophet) visited one of them and she said to him like this, whereupon he said: I have taken honey in the house of Zainab bint Jahsh and I will never do it again. It was at this (that the following verse was revealed): 'Why do you hold to be forbidden what Allah has made lawful for you... (up to). If you both ('A'isha and Hafsa) turn to Allah" up to:" And when the Prophet confided an information to one of his wives" (lxvi. 3). This refers to his saying: But I have taken honey. (Sahih Muslim)
In this version (which was also compiled by an-Nasa-i -see here and here- and abu Dawod -see here- in their Sunan) it is Zainab () who was the wife that gave the prophet () honey. And the wives referred to in the 3rd verse were Hafsah and 'Aishah ().
Both Muslim (see here) and al-Bukhari (see here) also quoted a version saying that the wife who gave the prophet () honey was Hafsah (). And the wives referred to in the 3rd verse woud be 'Aishah and Sawdah ().
Furthermore according to al-Qurtobi ibn abi Mulaikah has reported from ibn 'Abbas that it was Sawdah (). And Asbat reported from as-Sudi that it was Umm Salamah () (this was also reported by 'Ata' ibn Muslim).
Ibn al-'Arabi ابن العربي commented these different reports saying:
In the following I'm translating from Arabic. As these translations are of my own take them carefully!
ابن العربي: وهذا كله جهل أو تصور بغير علم.
All this is either ignorance or imagination with a lack (real) knowledge.
The second reason or statement is that he () prohibited for him the woman that has given or offered herself (in marriage to him) because of his wives. This view was held by ibn 'Abbas and 'Ikrimah and this woman would be Umm Sharik.
The third reason or statement was that he prohibited for himself (his slave) Maria the Coptic this was reported by ad-Daraqotni and seems to go along with the story reported by an-Nasa-i in his Sunan. Imam Malik also reported a similar statement, but with no connected narrator chain and in this hadith Maria was referred to as "Umm Ibrahim" (the mother of Ibrahim).
Al-Qurtobi further said the strongest (most authentic) one of these is the first and the weakest is the 2nd one and he quoted again ibn al-'Arabi:
قال ابن العربي: " أما ضعفه في السند فلعدم عدالة رواته، وأما ضعفه في معناه فلان رد النبي صلي الله عليه وسلم للموهوبة ليس تحريما لها، لان من رد ما وهب له لم يحرم عليه، إنما حقيقة التحريم بعد التحليل.
ibn al-'Arabi said (about the 2nd statement): as for the weakness in the narrator chain it is due to non trustworthy narrators, and as for the weakness in the content it is that the rejection of the prophet (Allahs prayers and blessings be upon him for the woman that offered herself in marriage is not a prohibition for her, as a person who rejects what was offered to him didn't prohibit it, as prohibition needs a prior permission1.
وأما من روي أنه حرم مارية القبطية فهو أمثل في السند وأقرب إلى المعنى، لكنه لم يدون في الصحيح.
As for what was reported that he prohibited for himself Maria the Coptic this better in the narrator chain and meaning, but it was not reported in the Sahih, but was reported as Mursal.
1: this means only after marrying this woman he could have declared her prohibited to him, but the prophet never married Umm Sharik.
You may also refer to tafsir ibn Kathir here.
Imam al-Qurtobi further added a few details on the topic by adding statements and narrations on the authority of imam Malik supporting the two stronger reasons. And explaining that declaring a wife as haram is not equal to an oath for the malikis (and doesn't make her haram without the intention of Dhihar or talaq) as it is the view of abu Hanifa. He also added statements saying that it is not allowed to anybody to prohibit something except with Allah and therefore in the madhhab of Malik and a-Shafi'i such a person doesn't need to perform kafarah if he declared for example food as haram while it would be if he declared a wife haram (and in case there was a group of wives only one kafarah is necessary).
Note that according to al-Qurtobi scholars have 18 different positions on how to qualify if a husband tells his wife "you are prohibited for me" one of them is that it doesn't count at all as if he has prohibited some food on him. As a wife is considered among the good things in life and Allah said:
O you who have believed, do not prohibit the good things which Allah has made lawful to you and do not transgress .. (5:87)
Finally note that al-Qurtobi said that the majority of scholars of Tafsir hold the opinion that this verse was revealed about Hafsah in whom's house the prophet () had intercourse with his slave in her absence.
Some narrations seem to support him as 'Umar informed a person (sometimes referred to as ibn 'Abbas) who asked him later about this.
According the statement of ibn al-'Arabi we have two possible reasons for revelation of the five verses:
- The story of the jealous wives who wanted to lift the prophet () a pleasure he had with anther wife (eating honey).
- The story of the wife who was displeased because her husband had (legal) intercourse with a slave (he owns) on her day in her house (bed).
in both cases the verses may apply and can be explained based on the storyline. In fact it is possible that both matters happened meanwhile or at least in the same time period before the revelation of the concerning verses so that both stories are included or addressed in the revelation, but Allah knows best.