Lechaliq Yotzah and Lo Savaru: Two Limudim from One Pasuk

Rabbi Zweig opens with two fundamental questions in the sugya of lo savaru eish on Yevamos 7a. First, Rashi (רש"י)'s position: the havah aminah was that one is chayav only when performing all thirty-nine melachos together. The pasuk of lo savaru teaches two things—that one melacha alone obligates a korban, AND that thirty-nine melachos incur thirty-nine separate korbanos. But how can the same pasuk teach both dinim? The Gemara (גמרא)'s method of deriving lechaliq yotzah seems to address the second point (thirty-nine is thirty-nine), while the din of carbon (teaching chiyuv for one) appears to address the first. The kashya is how to reconcile these overlapping limudim from identical source material. The second question relates to the Ramban (רמב"ן) in Chumash. The Ramban brings a midrash suggesting that without lo savaru eish, one might think havara is permitted on Shabbos (שבת) because it "warms the body" like refuah, analogous to milah which is permitted. But the kashya is: if the pasuk is needed to assur havara at all (le-gufey), how can Rav Noson in the midrash simultaneously hold that lo savaru teaches lechaliq? If the pasuk assurs havara, it cannot simultaneously be splitting melachos into separate categories—those two functions appear mutually exclusive.