That is quite funny.
I had forgotten about the alternative uniforms. You're right, Decker was
clearly visually identified as a captain when we first meet him, although I still think my theory is right that "the price" Kirk must pay for wanting his old ship back (and everything we know about in the Phase II series format documents assumes that Kirk is indeed a captain for the majority of the episodes, not an admiral-acting-as-captain) is a permanent reduction in rank and being stripped of the assumed privileges thereof. Basically, exactly what happens to him at the end of Star Trek IV, but self-inflicted rather than being as a result of an official hearing into his conduct.
Of course, we must further assume that if
Kirk did command a second five year mission between TMP and TWOK, and the ending of TMP certainly implies this, then his 'permanent' reduction to captain for a further five years was in itself only temporary, and a loophole was used to bump him back to admiral upon his second return to Earth.