It is however all to the good because we would surely make short work of discussion if we all had the same tastes and had nothing to do but agree with each other.

Very true.

I think Brandon’s tone is grave rather than cold when he confesses to Elinor. How his blaming himself for Eliza being headstrong cold, especially when she is?

Where in the novel does Brandon ever say that Eliza is "headstrong"? He essentially says that she was persuaded by a friend to roam around Bath and mingle with the wrong sort of people (in other words, one could argue that Eliza was a victim of peer pressure), and that the friend's father did nothing to attempt to control his daughter's and Eliza's behavior. I personally have never had the impression that Eliza was a generally disobedient child - only that she was a little naive, easily manipulated by Willoughby, and, yes, a bit disobedient and irresponsible during that ONE trip to Bath. That hardly indicates that being headstrong was her long-standing character flaw.

IMO, because in S&S2 we never actually see any interaction between Brandon and Eliza II, Brandon's comments about her seem rather cold and empty. I imagine that the omission of the scene from the screenplay where we see Brandon finding Eliza in London contributes to the problem, because without that scene, there is nothing to indicate that he truly cares for her. Jane Austen's Brandon says that Eliza was "a valued, a precious trust," but S&S2's Brandon never explicitly says that he loves her, cares for her, or that she is important to him in any way. His comments to Elinor do reveal that he is ashamed of his failures, but because he can't even put in a good word for Eliza, he almost seems to be bombarding Elinor with all of this unflattering information in the hope that she will feel pity and not judge him harshly. He is making excuses.

As able as this symbolism is I still find it wanting because there has never been anything wrong with Colonel Brandon.

So if we do fruit for fruit with their inadequacies in mind then the ripe fruit at Delaford symbolizes Marianne’s readiness for a real relationship with Colonel Brandon and Willoughby’s pitiful wild strawberries symbolized how unable and unwilling he was to have a real relationship with Marianne.