Wait, so you're expecting Google to implement a broad security policy where any recipient could claim the sender's email account by just listing the subject/contents of the emails they've previously exchanged?
If any of your suggestions were implemented and became part of Gmail's security policy, what are the chances that the attacker who (a) knew your password, (b) knew the answer to your security question, (c) went through the process of changing both, (d) switched the recovery email address (in other words, kinda knew what he was doing and seemed to have an experience with things of this nature) would competely and utterly forget to (f) send himself a bunch of emails, (g) set up the compromised email account as a forwarding address for an account of his?
I believe they do resort to additional verification methods if the location is suspicious, but in your case in absense of 2fa the only additional verification method seemed to be the security question. How secure was it? Are you sure the attacker didn't compromise your original recovery email account?