Which Defiant? Only the tier 5 retrofit ever had it. The tier 4 versions do not, being based on the production model Defiant-class (only the original Defiant had a cloak, not its replacement or any other canon ship in the class)
The Captain-level Tactical Escort doesn't, you're correct. And, per the Treaty of Algeron, the only Defiant-class ship that was equipped with a cloak was the Defiant herself, and it was supposed to only be used in the Gamma Quadrant (this was pretty much winked at after the Klingons went to war against the Cardassians in DS9).
That having been said, the Vice Admiral level Tactical Escort Retrofit (which, yes, is a C-store item) does come with a cloaking device console.
And, spoiler alert. The Rear Admiral level Advanced Escort (Prometheus-class and variants) don't have Multi Vector Assault Mode. Again, though, there is a Vice Admiral level retrofit that does in the C-store.
The Romulans no longer exist as the same political entity, and we are in open war with the Klingons.
Let me distill it down; on seeing the Borg Klingons Romulans and Dominion all against us the meeting of admirals must have gone something like this;
"Motion 1; **** the treaty. All in favor?"
*all hands go up*
"Very well. On to motion 2..."
Oh and there's the little detail of that executive order of the Federation President signed after the Hobus event stating the Federation still abides by this treaty despite the Romulans being regarded by the Federation as a new people.