If consensual homicide exists, why was there a need to place a reprimand on Worf's record for executing Duras?
Presumably because even though consensual homicide exists on Federation Members, it's something that can only occur in specific, regulated circumstances. Or perhaps because it is forbidden under the Starfleet Uniform Code of Military Justice. Or perhaps simply because a Starfleet officer engaging in a consensual homicide duel with a foreign political leader is considered a violation of the Prime Directive or something close to a Prime Directive.
Meta-textually, I'd say that the Federation's tolerance of the
kal-if-fee vs. Starfleet's intolerance of Klingon duels constitutes an inconsistency one way or the other.
At the end of the day, an interstellar state could not exist like a current day United States.
Of course not. An interstellar federation would by necessity have to delegate much more autonomy to its constituent polities. But that does not mean that the Federation is not a
federation, and it doesn't mean that the Federation doesn't guarantee certain rights and maintain certain requirements for its Members.
As far as Amok Time goes... T'Pau even says that the battle is for the possession of the woman T'Pring. And the episode indicates that she doesn't really have a way out legally if Spock wins.
Again, ceremonial language doesn't actually tell us that T'Pring will be considered her husband's chattel, because ceremonial language can be retained for purposes of tradition even wen they are inaccurate. We'd have to see actual Vulcan marriages for such info. The closest we've ever come would be the glimpses we've caught of Tuvok's marriage to T'Pel on VOY -- T'Pel certainly never seems subjugated to Tuvok, and she's definitely not in the TTN novels.
It is true that Vulcan arranged marriages would seem to contradict the Federation's claim to be based on "freedom and self-determination" ("The Best of Both Worlds"), though this particular imposition seems to fall on male and female Vulcans equally. I also question how much of T'Pring's plot was based on the need to appease tradition rather than legality.
And, of course, we don't know if Vulcan arranged marriage remain in place in the 24th Century just because they were there in the 23rd.
Then you add in Ardana from The Cloud Minders, who were Federation members and essentially treated a mentally-challenged subclass as slaves.
Yes -- and where the rest of the Federation, as exemplified by the
Enterprise crew, hadn't realized the working class was oppressed and reacted negatively to such a revelation, and invoked Federation negotiators to come in and start changing Ardanian society. I think it's safe to say that the Federation learned from the Ardanian debacle there; they're certainly much more careful and picky about admitting Members by the 24th Century.
I really think the Federation was suppose to be a loose alliance, probably primarily a defensive alliance not a collection morally like-minded folks.
At first, to a point. But the polities the Federation was modeled upon changed over time.