אכתוב-לו רבו תורתי כמו-זר נחשבו
seems to be missing something. The strange word רבו is traditionally read as רבי, multitudes, or as רבוא, myriad. Consequently, the Torah of HaShem is seen as constructed of out of multitudes, or of a myriad, ten thousand items. I would like to hypothesize the reading רבוע, a square. The square would be 160 times 160 words. The words רבו and זר are almost equal in Gematria, and therefore it makes sense that רבו is counted like זר. If we add a letter to one, we have to add the same letter to the second, רבוע is counted like זרע. Hence, in my view, the Pasuk must be read:אכתוב-לו רבוע תורתי כמו-זרע נחשבו
"When I write My Torah for him as a square, they will be considered like seed."It fits the prophetic context, and more or less answers the question, which we did not ask, why it says "for him." But, the Pasuk gives an answer to another question, which asks itself in the face of the utter destructon of the ten tribes of Yisrael. And the answer is "When I write My Torah for him as a square," and it is clear to who grasps it.
This would mean that "My Torah" is the combination of E and J, that we have called (my version of) the Torah Kedumah. This fits Hoshea, who in 12:4-5 refers to concepts of both E and J, and who in 9:10 refers to the J-story of Baal Peor (Hat-tip Friedman, though Hoshea 9 seems a later insertion, just as Hoshea 13:1-11). Moreover, fragments like 2:2 and 12:1 place Hoshea in the middle of E and J. See also this. He could have been RJE, as may be suggested by the Pasuk Hoshea 8:12.
