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.
With Taris out of the picture the last thing even pretending to be the government that signed the Treaty of Algeron is defunct. The Federation is welcome to continue to operate under its restrictions, but the existing Romulan government is not bound by it.