The EU (European Union) is Europe I can give you the definition of what it does if you'd like, started as an economic treaty to become more interdependent/prevent war. Has a delegation to the UN, so by that I would put them below the UN.
For the purposes of a strategy war game, you are getting way to hung up on the details. There are several Intergovernment Organisations, and they have different laws and boundaries, to say one is above the other, is speaking for the entire planet and i'm sorry but that is flawed reasoning. You won't get the UN overriding the laws of the EU any time soon I can assure you, things might be challenged on both sides from time to time however.
The UN, while a large organisaton, has very few controls over governments involved compared to say the EU, more guidelines and a general set of agreed upon laws, which many choose not to follow. There is no such option in the UP, once a law is passed, that's it I implement it or leave. I can't veto it, I can't go to other AI empires and offer them bribes to overturn it, I can't take a diplomatic hit for not doing it, there is no personal or strategic choice. It's be in a majority or get out. - It's much more like the EU than the UN.
I think from what you've said you are from America, speaking as a citizen who lives in europe and from my perspective, the EU is much more relevant in setting laws (much to peoples joy or ire), than the UN is, also in enforcing those laws. It is a much more relevant organisation in that regard, for the countries involved. So while the UN has more members it has much less influence or mindshare here.
The AU (African Union) is Africa, mainly to create a more stable solid Africa. As this is based in a region I put this along with the EU below the UN, however, they work with the UN they don't have a delegation to the UN.
The OIC (Organisation of Islamic Cooperation) Is a group to safeguard and protect the Muslim faith, help unify their voice. Have delegations to the UN and EU so by that I would put them below both of those groups.
I can't go into specifics of these two organisations as my knowledge of them is not sufficient, I would pick the Commonwealth of Nations (British Commonwealth) instead if you wanted another example, given that it is still expanding, it has a lot more power and influence than you give it credit for in your summary having 53 member states. You can't define it as a regional entity as it covers many parts of the globe, and you can't dismiss it as having nothing but trade in its mandate, it has a lot in its mandate.
Should I bring other regionals into it however?
Regional sure, as breakaway UP's would likely be in the game, smaller organisations of friends, allies or disgruntled UP members.
Now to your other points I would put them into either of the two groups where there isn't a great comparison in the game to match either the EU/AU or the OIC. Now could they add some type of governing party to go between the general races and the UP sure, is there really a need, no. Arguments about the UP being too strict or not agreeing would mean that you as race need to work to change that, to ease restrictions.
If you wanted to work with it sure. Playing the diplomatic race, great.
If you wanted to roll right over it with military might, that should be an option too.
If you wanted to bribe other races to get your way, why not let players?
If you wanted to ignore it completely and create your own group, why not, why not have that as an option? I still don't understand the fundamental concern here, it's not mechanics as i've covered nothing changes except the UP screen, it's not reality as i've covered that there are many different groups on planet earth even, let alone across a thousand star systems. Gameplay is enhanced by giving the player options not negated, and making diplomacy more unpredictable in a strategy game (unpredictable not confusing), is all good.
It wouldn't even take much more development as all it'd be as a carbon copy of the UP screen with different members, and 2 or 3 buttons on the interface. 2 should be sufficient the UP and it's competing sister organization.
So what's the concern?
There has been talk of having the miner races in the UP, being that as a whole they are small and have limited say maybe add a group to speak for the miner races similar to the OIC. Something where they can organize their voice to the large whole being they will never be a dominate race in the Galaxy. I also wouldn't mind something similar to smaller groups of larger races as well for regional or ideological views that may take on certain issues.
Overall though I still think one UP is the right way to go. They make work with several small groups and many individual races but is working to a overall better galaxy.
Minor races having a minor vote would be great. That would be very much like how babylon 5 had it
, with the chance they might become major through an event or with help from another major race, like me.
The ideology is a great idea and somewhat towards what I spoke of above, having another recourse diplomatically, especially when a group of races have no real influence compared to 1 or 2 powerhouses but share common goals, they should band together before a vote or after a vote to have it overturned. There should be a button on the diplomacy menu with those who share your ideology with all kinds of options, and this should be one of them.
I hope you're not taking anything personal as I'm enjoying this discussion even if we aren't agreeing
I enjoy a debate about a topic too as you often make discoveries you weren't aware of with an intelligent poster, keeping personalities or labels out of it helps keeps it on topic.