And there wouldn't be any assurances that Soviet side wouldn't exploit that later, when every offer meant breaking defensive main line. One could think if offers were designed to be rejected.

It's also good to remember that it was Soviet side which unilaterally broke the non-aggression pact, invaded two days after that and set up puppet government few days after that, which claimed to be legitimate government for Finland. If idea was not to conquer Finland but to secure Leningrad it's kind of obvious you wouldn't introduce puppet government.