1. This same point is made in the commentary to Job that Sara Japhet attributes to Rashbam, ad loc., and in Rashbam’s Dayyaqut. See note 51.

2. If, as I shall argue in note 51, Rashbam is claiming that ישך is (like ישל and ימל) a nifal form, it is hard to know how he understands the verse in Proverbs.

3. Saying that these words are ייפעל forms is not particularly helpful for us. In theory the word might be vocalized יִיפְעַל, meaning that all these verbs are qal forms. But Merdler argues, and I agree, that the word is to be read יִיפָּעֵל. Rashbam is claiming that the forms ישל, ימל and ישך are all nifal forms. The argument is made clearly by Rashbam in his Dayyaqut (Merdler’s edition, p. 12); see also the discussion in Merdler’s M.A. dissertation, p. 24 and note 140 on pages 107-108. (On page 24 of the dissertation, line 11, read, I presume, ואת נטיית פ"נ בנפעל עתיד, not ואת נטיית פ"נ בהפעיל עתיד.)
In his notes to Ibn Ezra’s commentary to our verse, Weiser says that Ibn Ezra errs when he describes the form ישל here as nifal. Weiser then cites Rashbam’s commentary here to “prove” that the form is qal. However, if we follow Merdler’s analysis, Weiser, and not Ibn Ezra, is the one who errs. Ibn Ezra and Rashbam both offer the explanation that ישל is nifal. This explanation admittedly does not conform to today’s theories of Hebrew grammar but need not be seen as a “mistake.” Rosin (note 15 here) also understands Rashbam’s comment here as a claim that all of these forms are nifal. (Rosin, in turn, takes issue with Rashbam’s claim).
As Merdler notes (note 10 on p. 12 of her edition of Dayyaqut), Rashbam apparently contradicts himself in his grammatical commentary on Genesis, appended to his Dayyaqut. There he seems to see some of these forms as qal. See comment on Genesis 2:7 on p. 56 of her edition of Dayyaqut. In her (1995) M.A. dissertation (p. 24) Merdler also noted the contradiction and suggested a harmonizing emendation of the passage from the grammatical commentary (reading ייפעל instead of ויפעל). In her (1999) edition of Dayyaqut I find no mention of the emendation.
As I noted in note 44 to Deuteronomy 7:1, one of the reasons that Rashbam might like to see ישל here as a nifal form is that he is convinced that the qal of the verb נ-ש-ל is always transitive. See also note 45 there and note 39 to Deuteronomy 19:5.
Moderns generally see all these forms as qal. See e.g. BDB.