Fair enough. My general rule is to assume that if a member of a species is in Starfleet, his world is probably a UFP Member, but, as you note, The Entropy Effect establishes otherwise, so off the list it goes.
See, my own take on TMP aliens is that several had joined the UFP between TOS and TMP, and that many had repesentatives serving in Starfleet, perhaps starting off as exchange officers (and then "Ex Machina" has a subplot about Dr Chapel's role in assisting the new UFP races recruits adapt to Starfleet life on the Enterprise).
When I chatted to TOS/TMP makeup man Fred Phillips in 1983, he mentioned that the expensive alien ambassadorial costumes were ordered up because there was a plan to have Nogura addressing a "Journey to Babel"-type banquet in which they are told of the gigantic cloud heading for Earth. That idea didn't progress because Robert Wise found most of the masks wanting, and relegated most to background scenes, where little detail would be evident.
To my delight, a fun character in "The Lost Years", described as an Arcturian, has the same physical description and voice patterns of the alien McCoy meets in the bar in ST III. If you put that guy and a TMP cloned Arcturian next to each other, they have remarkably similar noses. Adding to that, a humanoid female is wearing the leather TMP Arcturian outfit in ST IV's council scenes.
I'd love to see a future novel address the Arcturian cloning issues in light of canonical comments that have arisen in TNG, and all the anti-genetic-tampering from DS9.