The gemara makes a contrast between one who is מתעסק in חלבים ועריות and one who is מתעסק in hilchos shabbos. Regarding חלבים ועריות the rule is that even a mis'aseik is chayev because he is receiving hana'ah. The principal is that although one who is misa'seik is generally exempt and not regarded as doing an issur, somehow the pleasure that they receive from the action allows us to attribute the action to them and make them liable. The gemara contrasts this to hilchos shabbos where mis'aseik is exempt because מלאכת מחשבת אסרה תורה. Tosafos raises a fundamental question, why does the gemara cite the special rule of shabbos of mi'leches mach'sheves which indicates that intent plays a more significant role in hilchos shabbos, rather than simply say that in all other issurei torah (except for when one receives hana'ah), mis'aseik is patur? Tosafos offers 2 approaches. Their second approach is that there are 2 types of mis'aseik, one which is exempt from the standard pasuk of mis'aseik but is considered mileches machsheves because he accomplished his goal (i.e. trying to pick up a vegetable thinking it was detached and then realizing that it was attached). The other which is exempt based on mileches ma'chsheves such as when one tries to pick one vegetable and ends up picking another. However, in Tosafos first approach they offer a very interesting answer:
והכי קאמר, בשבת פטור אף על פי שנהנה משום מלאכת מחשבת
Tosafos is saying that the standard exemption of mis'aseik doesn't apply to a case where one benefits, as we see by חלבים ועריות where one is chayev even as a mis'aseik because of the benefit they receive. Therefore, when one violates shabbos as a mis'aseik but receives a hana'ah in the process, they should be chayev if not for the fact that the Torah has a special requirement of mileches machsheves.
This approach of Tosafos only makes sense if we assume that the concept of שכן נהנה would apply to hilchos shabbos and be mechayev one who is mis'aseik. However, it seems that the concept of hana'ah being mechayev even one who is mis'aseik doesn't apply to hilchos shabbos. The gemara picks 2 examples of where שכן נהנה applies, both eating cheilev (animal fats) and arayos (forbidden relations) because the nature of the prohibition is an issur of receiving a forbidden benefit. Within the realm of issurei achila, the rambam considers the general concept of issur achila to be an issur hana'ah (sefer hamitzvos #187 to explain why achila and hana'ah of meat and milk aren't counted separately), just that the Torah only assurs a very specific type of hana'ah, eating. Similarly, arayos is in essence an issur of receiving pleasure from a relationship with someone off limits. It is by these types of issurim where we say that the hana'ah can make up for the lack of awareness and be me'chayev one who is misa'seik. But, in hilchos shabbos where the nature of the prohibition is an act of melacha, rather than an issur of hana'ah, why does Tosafos assume that the pleasure he receives would make up for a lack of awareness to be mechayev one who is mis'aseik?
Furthermore, R. Akiva Eiger (teshuvos) assumes that the concept of שכן נהנה would apply to wearing sha'atnez and be me'chayev one who is mis'aseik in the wearing of sha'atnez (doesn't know it is ke'laim). The Imrei Bina (Dinei Shabbos end of siman 7) asks on R. Akiva Eiger:
ולא זכיתי להבין דבריו, הא כיון דשמואל נקט רק חלבים ועריות משמע דדוקא באלו דהוי אכילת איסור דנכנס תוך הגוף ועריות שפועל כל הגוף, בזה חייב שכן נהנה ולא בשאר הנאות
The Imrei Bina assumes that it is only a certain type of hana'ah, one that affects the entire body, would qualify as שכן נהנה to be mechayev a mis'aseik. His source for this limitation is the choice of חלבים ועריות as examples. He further proves this from the Rambam in hilchos shegagos (2:7) who writes the the שכן נהנה concept applies to arayos and forbidden foods, clearly indicating that it doesn't apply to other pleasures such as one who is mis'aseik in smearing themselves with שמן המשחה, since the benefit is minor they would be exempt.
To me it seems that the Rambam is not coming to the exclusion of more minor benefits, but rather to the exclusion of cases where the nature of the issur is an issur ma'aseh, not "pleasure". The Rambam holds that only in regard to arayos, ma'achalos asuros and things similar where the nature of the issur is to prohibit pleasure or benefit, are we mechayev a mis'aseik שכן נהנה. But when the nature of the issur is not benefit, rather an issur to do something, such as melacha on shabbos, even one who receives pleasure would be exempt as a mis'aseik. This will answer his question on R. Akiva Eiger. Sha'atnez is an issur hana'ah. It is not a typical issur hana'ah because only certain types of hana'ah are assur, but it is an issur hana'ah to be warmed by the garment, therefore one who receives this hana'ah would be chayev even when they are mis'aseik.