I confess to being puzzled that 23rd Century Ardana is being held against the 24th Century Federation vis a vis Bajor. The entire point of "The Cloud Minders" was that the Federation was going to force Ardana to change, and 103 years passed between TOS Season Three and DSN Season Four. For all we know, the Federation Charter was amended to ban caste-based discrimination precisely because
The way it was said, T'Pau turning down the seat was a unpresidented move. More than her simply being offered the seat and saying no thanks. if only a select few from the general assemblage are offered seat on the upper council, with the original founders having the offer as a given, then T'Pau's refusial to move from the general assemblage up to the upper council might have been shocking. Maybe the only time Vulcan was never on the upper council in the Federation's history.
There is no evidence whatsoever that the Federation has a bicameral legislature. Canonically, there is the Federation Council, the Federation President, the Federation Supreme Court, and the President's Cabinet. That's it.
Once Ardana was in, there wouldn't have been much the Federation could do, especially if the individual members are in fact sovereign entities.
Actually, full sovereignty on the part of Federation Members is the only way there could be "not much" the Federation could do. If the Federation itself, however, is the sovereign, then the Federation would have plenty of options for forcing Ardana to change. Just ask the Southern states that were forced to abandon Jim Crow what a federal government can force from its member polities.