I'm not sure. From TOS onward, we no longer see any indication that the Vulcans, Tellarites, and Andorians had their own fleets, at least non-Earth style vessels. We know that the Vulcans had transport craft, but it generally doesn't seem to make much sense that each of these worlds would continue to build their own ships after the founding of the Federation. It makes more sense to have one singular Starfleet charged with exploration and defense of the various member worlds/systems.
I don't see how that makes sense, because the Federation is just that, a confederation of sovereign nations cooperating for mutual benefit and defense.
Erm.... No. If it was, it would be called the "United Confederation of Planets." That it is called the "Federation" means it is exactly that -- a federal state.
The UFP displays all of the traits of a state, to boot. It has its own territory; it has a legislature capable of making Federation-wide law (eg, the Warp 5 speed limit from "Force of Nature"); it controls foreign policy towards non-Federation polities; it has a well-defined bureaucracy that's quite extensive; it holds a legal monopoly on the use of violence; its government is capable of instituting martial law on Member States' territory without needing to consult that Member State; it has its own military.
The United Federation of Planets is rather plainly a sovereign state in the same way that Canada or the United States or the Commonwealth of Australia are.
It's not the United States writ large. It's more like the European Union (though not exactly, just relatively).
Hmmm. Gonna have to disagree with you on that. The EU hasn't really taken its final form -- Europe's still going back and forth on EU statehood. But while the Federation's Member States may have more autonomy than US states, it's pretty clear that the Federation is a federal state in its own right, just like the US or Canada or Australia.
All this to say nothing of the fact that the Federation has rather explicitly been described as a nation in its own right in novels like Articles of the Federation