1) Rav Margoliot, in Sefer ha'Mikra veha'Mesorah, writes that Rebbi Tarfon changed the verse from its original form in order not to trangress the prohibition of saying, orally (Ba'al Peh), a verse written in the Torah. On page 47 he cites the Gemara in Eruvin (65a) which states that one should not pray when in distress, as is says, "ba'Tzar Al Yoreh" (literally, "one should not instruct when upset"). Rashi writes that he could not find this verse anywhere in Tanach. Tosfos (end of DH ba'Tzar) cites a verse which has some similarity to "ba'Tzar Al Yoreh" and adds that the Gemara in several places is accustomed to shortening the verses from their original form. This rule is also stated by Tosfos in Shabbos (128a, v'Nasan) and in Megilah (3a, DH va'Yalen).

2) The implication of Rav Margaliot is that the reason why Tosfos writes that the way of the Gemara is to shorten the verses is to avoid the prohibition of saying written Torah by heart. In fact, I found that this idea is stated in the introduction to the Teshuvos Beis ha'Levi, near the end of the first paragraph DH Amar. In ha'Mikra veha'Mesorah, Rav Margoliot does not cite the Beis ha'Levi but his Pshat is very similar. The Beis ha'Levi writes that when they used to explain or expound a verse in the times of the Gemara, they used to read it from a scroll. Sometimes, if a scroll with the verses written in it was not available, they would say the verse in a way slightly different from the orignal form.

3) Rav Margoliot writes, on page 49, that this is what happened in Zevachim 62b. Rebbi Tarfon was sitting with his nephews and they kept quiet, so he tried saying some words of Agadah. He did not have a Sefer Torah available but he wanted to explain the verse about Keturah, so he deliberately changed it slightly. The reason he used the word "Yochni" is that this is a word often used in Agadah (see Menachos 85a, and Rashi DH Amar Lei Yochana). The nephews thought that their uncle had made a mistake so they corrected him.

4) Rav Margoliot explains, with the help of the Gemara in Yevamos 61a, why Rebbi Tarfon used the word "Keturah." The Mishnah describes somebody who paid money in order to become the Kohen Gadol. Rav Yosef said, "I see a 'Katir' here." This means literally "a knot," refering to a wicked plot. Rav Margaliot writes that whenever someone is unworthy for a position, this is termed a "Katir." The same thing applied to his nephews because he could see that they were not worthy of discussing Halachah.

5) The Sefer Tuv Ayin explains, in a similar manner to Rav Margoliot, how the Sugya of "Devarim she'Bichtav Iy Atah Rashai l'Omran Al Peh" is connected to the passage at the beginning of Zevachim 62b. The Sefer is by Rav Eliezer Moshe ha'Levi Horowitz. At the beginning of the Sefer, in the approbations, there is a Haskamah from Rav David ha'Levi Ish Horwitz, who explains as follows:

Rabbi Tarfon did not want to say the entire verse, "And Avraham married again...", because of the prohibition of saying written Torah orally (Ba'al Peh), so instead he said only part of the verse. This is why he said her name was Yochni, rather than Keturah, in order to avoid saying a complete verse. Rebbi Tarfon hoped that his nephews would understand the hint and possibly a discussion would develop about saying orally the verses of Torah. Regretably, the nephews did not understand what their uncle was talking about. This is why Rashi writes that they do not know how to speak about Halachah.

Why did Rebbi Tarfon chose specifically the word "Yochni" instead of "Keturah"? Rav Horowitz cites the Gemara in Shabbos 54b where Rav Huna said that there is a tree by the name of "Chanon" which grows in the islands. This tree has a pleasant smell. "Chanon" and "Yochni" are similar, so Rebbi Tarfon was hinting at Keturah, which also has a pleasant smell.

Kol Tuv,

Dovid Bloom