I dont think this is true. I imagine the designers of the EU were fully aware that foreign govs might renege on deals and built mechanisms to deal with that.
For the most part, the rules based international order is built around the idea that following it is in everyone's best interest and going against it will have negative consequences. That is why it has lasted so long.
Yes, that is what people mean by a rules based international order.
Its not literally followed rigidly, but there are certain norms that are followed 90% of the time, and that makes things more predictable which is a good thing.
How often are diplomats executed to send a message to there home country? Basically never. That is the system. Its far from perfect, definitely not comprehensive, but does do some good every now and then.
So you think diplomats are executed to send a message, there is just the appearance of them not being?
Again, what is meant by a rule based order is not that there are solid rules upheld consistently as in criminal law, just that there are norms which are usually obeyed.