That's quite a stretch really. To answer your question, Enterprise has "Photonic" topedoes (yeah, the name is extremely stupid). So, when they fired them at the Klingon ship, the Klingons knew it was some kind of anti-matter weapon (like a Photon Torpedo is), but didn't know the exact type (Photonic).
The Romulan language question is simple, either the Romulans changed their language when they left Vulcan, or the Vulcans changed their language with the reformation, or both. In Gambit, Picard clearly says that the artifacts he was working with has an alphabet more consistant with early Vulcan than early Romulan. Aside from that, I can't imagine anyone translating a language without extreme difficulty.
The cloak thing had been admited as a mistake. They should have known better, but they messed up. The only explanation I can think of right now was the exact phrase used by Spock (Practical invisibility). The series has depicted an evolution of technologies where one side creates a cloaking technology, and the other side finds a counter-measure (Geordie's Tachyon field, Kirk stealing the cloak to examine it better, etc.). It is possible that this Romulan cloak is not even close to practical by the 23rd Century, and the outpost's sensors should have been able to easily see through it (and they couldn't understand why they weren't able to). It might be a stretch, but it's better than thinking Kirk didn't know about Romulan technology 100 years ago (which doesn't sound like a popular idea).
Here's another thought, the Treaty of Algeron was mentioned by Data (in the Defector) to have established the Neutral Zone, which was created at the end of the Romulan War. In "The Pegasus", the Treaty of Algeron was said to specifically prohibit the Federation from using cloaks, which would suggest that cloaks could exist during the Romulan War.
My major complaint with the Romulan Cloak in Minefield wasn't the fact that they had it, but how it was used. In Balance of Terror, the cloak was very power intensive, and was part of the reason their mission failed (they ran out of power). But, in Minefield, it was used for no reason at all, which suggests power wasn't a concern.