As neat as that map is, unless you show me the episode it appears in... it doesn't count in regards to a canonical argument. T'Girl is right about the Klingons not sharing a border with Cardassia. Not only would they need a shared border to invade and hold worlds as they did in season 4 of DS9(unless you think the Federation that fought a conflict simultaneously with them just consented to supply lines through their space), there was the Betreka Nebula incident that Bashir and Garak discussed, a conflict between the Klingons and Cardassians that lasted 18 years.
And your argument about the Warbirds being less powerful than the Sovereign really doesn't hold water either. Let's remember Shinzon's objectives in that battle. He wanted to capture Picard, so he had to be somewhat selective in his targeting. Not to hit a critical power system or anything. Not to mention his tendency to toy with Picard the entire movie. He could shoot to kill against the Romulans with none of these factors applying.