Regarding your last point, it is impossible to compute a hash of the HRR and to store that inside a cookie, as changing the cookie would change the HRR and the client would compute a different HRR hash than the one stored inside the cookie. Leaving the client and server with different transcripts...
Like you, I don't understand how that construction makes it possible for the HRR to be stateless. I don't even understand what the added security of keeping the ClientHello1 and HelloRetryRequest inside the transcript-hash instead of "simply" starting anew with ClientHello2.
The RFC is explicit that the transcript-hash shouldn't be reset but there is no explanation why
From section 2.1
Note: The handshake transcript incorporates the initial ClientHello/HelloRetryRequest exchange; it is not reset with the new ClientHello.
I guess we have to implement that silly construction and get none of its advertised benefits...
