The proposal was needed because when it works, DS is about human beings and not just plot (what physically happens next). Victoria leaving was bound to strike Barnabas as the very last chance he'd have with her, or with anyone since he had idealized her as he had Josette. (He does that too easily, but he doesn't know that.)
I'm never sure if we're really seeing the character of Barnabas develop, or if we're just reading that into it, but this seems like a pivotal moment. It brings out just where Barnabas is in his head at this point. He's a bit like a child rediiscovering the world now that he's alive, and he presses Vicki at exactly the wrong moment for her. Obviously she wouldn't be thinking of leaving in the first place, if she thought that way about Barnabas at all. He makes things about him, and when she rejects him, he's probably surprised to find himself adjusting and being big about it (eventually). This is a change. It paves the way for him rescuing her only to let her go with Bradford, later.
I agree, AM could always have done the character better. (And the proposal would have meant more.) I do think, though, that the semi-hysteria was in the script and was a reaction to overwhelming loss (we don't think much of Clark but she does). Moltke would have had to play it the same way... she couldn't just choose not to be so upset. It was written that way.
BD's Victoria was living a different situation than AM's was, so she's bound to come across differently. We all do when faced with new situations we're unprepared for.