Cleansing is basically free for advanced civilizations in the books. The alien (Singer) who wipes out Sol in the 3rd book doesn't even have to answer any questions from their manager about doing it, that's how cheap it is. While its true that individuals desire cooperation, I think you can assume that civilizations will keep a lid on people who will completely destroy them (or failing that, be destroyed). It seems like expansion of civilizations is not really an option. The Singer's civilization only has 1 colony world and they're already in some kind of extremely destructive war with them. Presumably the idea is once your own people expand multiple light years away, all the logic about aliens applies to them too. On the other hand if you can't expand why do you not run scorched earth on the galaxy?

There definitely is some weirdness about observation and communication: Singer's civilization can wipe out Sol with a flick of the wrist, but while they can observe the number and type of Earth's planets, that seems to be their limit. The sophon enables FTL communication and observation between Earth and Trisolaris, but the more advanced civilizations don't seem to make use of them? You could be absolutely certain of someone's threat level and intentions with one. Maybe something about the technology can be traced back to its origin system, so they are too risky to use.

I think it's all reasonable in the books, especially as a self-reinforcing state. It does definitely require a highly specific set of universal laws / technological constraints though. If the FTL drive didn't also broadcast your position to the whole universe for eg, it would crack everything wide open.