You are asking a very interesting question.

Many scholars have studied the subject of Rashi's commentary on Perek Chelek, and many articles have been written. However, most of them deal with various proofs as to whether the commentary before us is not Rashi's commentary at all, but rather someone else's (some say it is by his son-in-law, the Rivan, and some say it is by others), or whether it is a mixed commentary by Rashi and someone else.

There are two types of evidence. There is evidence that stems from familiarity with Rashi's words in other places. The author of Dikdukei Sofrim insists that this is not Rashi's commentary because the style of this commentary does not match his familiarity with Rashi's style. There is another type of evidence, which is examining many other books that quote this commentary and examining whether the commentary is quoted in the name of Rashi or someone else. It turns out that already in the time of the Rishonim (like the Yad Ramah and others), they were quoting the commentary before us in the name of Rashi. So, even if those who say that this is not Rashi's commentary are correct, this mistake is already over 700 years old, and it is difficult to trace and know how it happened. Was Rashi's commentary really lost, or did someone edit the commentary and mix in other interpretations? I am not aware of a clear decision on the subject, beyond the articles that have been written at length about the mere identification of the commentary.

Regarding the Tosfos. It is known that the source of the Tosfos in all of Shas is not uniform. There are different sources for the words of Tosfos, and this is a lengthy topic in itself. Regarding Maseches Sanhedrin, it is customary to say that the source of the Tosfos is from the Talmidei Rabeinu Peretz, who copied and edited the Tosfos Shantz, or from the words of the Rash mi'Shantz. We see in many cases of Agadeta in Shas that the Tosfos are fewer in comparison to issues of Halachah. It is reasonable to assume that in the case of Maseches Sanhedrin, the source of the Tosfos is a Ba'al Tosfos who did not deal with the matters of Agadeta at all, as we have found in other Rishonim such as the Rif, the Rosh, the Ran, and others. I have seen scholars speculate as I wrote, but as I said, this is just a hypothesis.

I hope this helps,

Aharon Steiner