If you don't know of the existance of another race, and they don't know about you, how could you meet to vote on things?

If they were all at the UP and you're talking to them, you've discovered them, right? Couldn't you just go up to the ambassador and say, "Hey guy. Where in the galaxy do you come from?"   

Personally, I think the way the game plays makes perfect sense. Just think of the UP as an organization that you decide to form with the civilizations that you have met (and not some galactic agency that somehow exists apart from all civilizations). Then it makes sense that you and the one other guy on your side of the galaxy would form the organization and you wouldn't invite folks from the other side of the galaxy that you've never met.

I kind of agree with number one. Just assume that the voting isn't taking place in one physical location--you enter your vote from your homeworld, and the other races do the same. You should see the other races in the UP, and not as just a "?" either. I mean, you've known from the start of the game what other races are around. Heck, you can even see their territories on the map. You may not be able to talk to them, trade with them, or spy on them until you've met in person, but you still know they're there.

As for #2, they have that now. You don't NEED to add extras to the ships. In fact, I'm of the opinion that some of the ship hulls already look better with virtually nothing added. Just stick your guns and engines in pretty places on an empty hull and away you go. You've got 5 sets of hulls to choose from. If you don't like the one that comes with your race, change it. Some of them (Yor and Torian especially) are quite fancy, some (Human and Drengin) are fairly simple, just take whatever you like best.

Question: If UP convenes without all (potential) members present and a law is passed, does the law apply to all future members as well; and if so, then does the bill take effect immediately for those races, or only after they've joined the council? (ie. you make contact)

It should be the races that the Terrans have discovered. If you look at the history in Databanks, you'll see that Lord Wardell has written that it is the humans who start the United Planets. You shouldn't be able to vote without meetin the terrans first, though that raises questions if you are playing a game without the Terran Alliance. I also think that a "Have Contact With ....." should be avaliable in the diplomacy screen, like in Civ3. That would make more sense. Thats just my opinion, though.