I don't really think that either of these needs to happen. (Although I admit that the second suggestion is interesting. It has its own problems, though. Like: Who determines the state of the relationship?)
First: Federations allow a group of small realms to play the big game. I wouldn't want to restrict the flexibility to the extent that two-realm federations would do.
Second: I think that the act of federating may need to be more transitive, and far-looking when signed. It needs to do a recursive look at relationships when accepting a new federation partner. If it is possible, through the new relationships that a federation will create, to trace a Federated-or-Aligned path to someone with a War or Hatred status, the federation should be impossible, regardless of the distance of that relationship. This restriction already applies, but I don't think it looks far enough when accepting new partners into an existing federation. We had a situation on EC where Perdan was allied to Westmoor and at war with Ibladesh, while Ibladesh and Westmoor were Federated. There's a bug about it on the bugtracker somewhere... If these relationships were further traced out to look for conflicts, then this kind of thing would not happen.
The situation you dislike, I disagree on. I see it as good as it allows conflict to still happen. It seems like the people problem of people going into large federations/alliances forever is more or less impossible to stop so I would rather have a situation that allows for war to still happen.
Personally I hate federation status as it encourages peace a hell of a more than alliance. You were considering to betray a federated partner? O that sucks, you have to fight all of your current federated partners than.
As I said above, I am definitely for requiring everyone in the federation to agree to the new partner, not just one realm.
Well, I don't know about the supposed to happen (although I suspect you are right from what I have heard), but I must admit that it is hilarious. Tara gets a new federated partner, then get a double portion back on their plate. Classic.
Its not funny to me. All that happened was someone figured out how to use game mechanics to forcefully end a war. I didn't complain when it happened with Carelia joining the federation about them not getting a choice, because I thought they did, which then surprised me we made got into the federation. Now I know thats not true, so I don't care for that either.
Personally I think federation status should not be group-connected. If people want to make a group federation, go for it, but I see no benefit to the current group-connected federations vs just keeping it like the rest of the relations.