Question:how does the attribute of kel rachum vechanun tell you that the prayer of 13 middos will not be returned empty handed. it seems to be a strange "hemshich". or am i reading it wrong......
The SIFSEI CHACHAMIM asks your question, pointing out that Rav Yehudah seems to be making a Derashah with the introductory words, "Kel Rachum v'Chanun," but then he concludes with the words of a later verse.
The simple understanding is that when Rav Yehudah says that "a covenant has been established for the 13 Midos, that [their recitation] will never return empty (unanswered)," he does not derive that from the words "Kel Rachum v'Chanun" (Shemos 34:6), but from the words that appear four verses later, "I hereby establish a covenant" (Shemos 34:10), which refers back to the 13 Midos mentioned in the early verses, as Rashi here mentions. The words "Kel Rachum v'Chanun" do seem out of place in the Gemara (the Torah Temimah omits these words when he quotes this Gemara). It seems that these words are either being quoted by the Gemara as a transition to Rav Yehudah's statement, as the Gemara is explaining the phrases of these verses and the relevant Derashos. Indeed, in the Girsa of the Dikdukei Sofrim, the phrase continues with the rest of the verse, "Erech Apayim v'Rach Chesed v'Emes."
(Alternatively, perhaps these words are all part of Rebbi Yochanan's statement, showing what the Seder is of the "Seder ha'Zeh" of the 13 Midos which Rebbi Yochanan mentioned.)