Well for me personally I consider "canon" to be ultimately the intent of the creators rather than what appears on screen (for the most part). Otherwise all the retcons Star Trek has would be difficult to explain.
But writer intent in Trek is typically far more obscure and contradictory than the end result - because for every aired adventure, there are a dozen conflicting ideas, from the writers of that episode, from the writers of earlier episodes that this one derives from, and from the writers of later episodes that derive from this one. What we see is the refined outcome of all this mulling.
Having two Ambassadors
in "Redemption II", rather than one that changes configuration in mid-flight, seems simple enough an interpretation of what we actually see... Anything only seen behind the scenes, or in a book somewhere, exists in a separate universe that should not be allowed to confuse us. After all, the real
writer intent is to get money anyway, and money isn't part of the modern Trek universe.
Admittedly, in that model of viewing Trek, we can't even tell what the registry number of the Horatio
might be, so whether she's first batch, second batch, third batch or the rare pink three-and-a-half-nacelled variant is left even more obscure than otherwise.