Mesasek BiShabbos

The Gemorah in Keritot (19b) brings that Rebbe Eliezer has a derash of אשר חטא בה פרט למתעסק.  Rebbe Yehoshua understands that derash tells us that one must know what sin they are bringing the korban for.  On the next page, the Gemorah brings a statement from Rav Nachman in the name of Shmuel that מתעסק בשבת פטור מאי טעמא מלאכת מחשבת אסרה תורה.  Rashi explains that this reasoning of מלאכת מחשבת אסרה תורה is only needed for the opinion of Rebbe Yehoshua but according to Rebbe Eliezer, the exemption is the general din of אשר חטא בה.
Tosfos in Sanhedrin (62b) understands that both dinim are necessary.  One approach in Tosfos is that we need the din of מלאכת מחשבת to tell us even when the person gets benefit from the מסעסק action, which doesn't fit under the exemption of אשר חטא בה as the first half of Shmuel's statement says המתעסק בחלבים ועריות חייב שכן נהנה.  [There are two ways to understand this answer of Tosfos.  Either he means I need מלאכת מחשבת for a case where this is physical pleasure, or all מלאכת שבת is considered שכן נהנה for there is a positive, constructive act. (See Maharam and Achiezer volume 3 #27.)
The second answer of Tosfos is that there are two cases of מתעסק.  One scenario is where for ex. one inteded to cut cut grain that he thought was detached from the ground ,but it actually is attached.  the person's intent to cut this grain was fulfilled and the exemption from חטאת is only from אשר חטא בה.  The case of the exemption of מלאכת מחשבת is where one intended to cut one grain and ended up cutting another one. (Seemingly that case should also have the exemption of אשר חטא בה, but its also exempt because of a lack of מלאכת מחשבת.) [There are various variant texts in Keritut (19b) and according to some Rishonim this is explicit in the Gemorah.]
The Gemorah in Keritot says that says according to Rebbe Shimon that holds מקלקל בחבורה is obligated, מתעסק by חבורה is obligated as well.  Tosfos in Shabbos (75a) explains the comparison is both have the same exemption; מלאכת מחשבת and we say from מקלקל that there is no such exemption regarding חבורה.  Ask the Achronim, according to Tosfos there should be another exemption of בה as well?
Some Achronim understand that the Rambam holds that אשר חטא בה functions as a גילוי מילתא that מתעסק is not considered a מלאכת מחשבת and is exempt from a חטאת.  In other words, its only an exemption regarding Shabbot, but everywhere else מתעסק is obligated.  (See Perush Hamishna in Keritut, Ch. 2 law 7 and Ch. 7 law 11 of Sheggagot.)  According to this, שכן נהנה is לאו דוקא but in reality מתעסק is always exempt except for regarding Shabbos. [The Meiri in Shabbos and Pesachim sounds like this as well.]