M'cheini na m'sifricha

Moshe Rabeinu argues that if Hashem will not forgive Bnei Yisrael, "M'cheini na m'sifricha", to which Hashem responds, "Mi asher chata li emchenu mi'sifri". What was Moshe's "hava amina" and what was the maskana?
The "hava amina": 1) Ramban suggests that Moshe's claim was that if BN"Y were guilty, then he as their leader shares responsibility for that guilt and should also suffer the consequences. 2) The Seforno writes that Moshe asked that some of his zechuyos be transferred to BN"Y to mitigate their punishment.
The maskana: 2) According to Seforno, it is categorically impossible to transfer zechuyos from one party to another - everyone must succeed or fail based on his/her own merits. 1) More difficult to understand is the Ramban - do we not subscribe to the notion that a leader is responsible for the dor and can even be punished in their place? Was this not applicable to Moshe, and if so, why not?
I have not found an explanation for the repitition of possessives in the pesukim: Moshe stresses "m'sifriCHA and katavTA" - YOUR sefer that YOU wrote (which may be because Hashem himself inscribes tzadikim to the sefer hachaim, assuming that is what is being referred to). Hashem's reply stresses "mi asher chatah LI" - who sinned against ME, "emchenu m'-sifRI", will be erased from MY BOOK.