I think the GTVA's strategy with containing a Shivan invasion is quite good, and they did successfully enact and carry out their contingency plan for evacuating an inhibited star system in the Second Great War. What bothers me more, at least in the BP backstory, is that Capella seems to have magnified the GTVA's weaknesses as a governing body. This can be seen in GTVA's performance in dealing with the aftermath - slow economic recovery, increasing regionalism and strained diplomatic relations with the Vasudans and now the UEF civil war. Reduced Terran-Vasudan officer exchanges, for example, hampers coordination between fleets in case of an invasion. Continuing economic problems makes the GTVA a less attractive government, contributes to secessionist activity, and may hamper fleet building. These deficiencies are already undermining and will continue to undermine the GTVA's ability to coordinate its activities and deal with the Shivan threat and other threats to humanity, and, depending on the severity of these issues, may eventually threaten the GTVA's continuation as a political entity. Success is more than just a ideological choice between 'vigilance' and 'negotiation' but also a matter of expediency and effectiveness in government. In light of repeated Shivan invasions, a test of good governance is that the GTVA recovers from crisis, not just stopping the incursions but thereafter regaining strength socially and economically. Whilst returning to Sol held the GTVA together, even if they win the civil war, it doesn't exactly fix the long-terms diplomatic and governance issues.
It's possible to take a "well, if humanity is dumb enough to reject the GTVA, then it deserves to end" kind of opinion, but I don't think it's that simple.
I do not propose the UEF as an ideal solution to these problems. I agree with the GTVI opinion that the UEF is a product of its circumstances, and if it were to take on the GTVA's role, it would be a very different animal. Its military preparedness is probably not at GTVA levels, and that is a definite problem. However, I find the GTVI's view on the threat of mass emigration to Sol quite extraordinary - is the GTVA that bad at serving its citizens needs or Ubuntu that good? The UEF's amazing success at realigning Sol's economy and socio-political situation after losing all interstellar contact is quite incredible, the amount of redistribution of resources in the economy, for example, would have caused disturbances fatal to many governments (the Sol GTA's failed command economy being one example).
As for the talking to aliens situation, I think I'll have to see WiH R2 first. We know these people exist - the Beis, Noemi; there are other, possibly - the prophet, the Elders, Lt Ash, FS1 Alpha One, Bosch, maybe others. We'll have to see. I think the resolution of the situation also depends on what the Vishnans meant in the dialogue with Bei at the end of AoA as well.