The current system was created by the winners, naturally. Which included China (ROC) and the USSR.
Regardless of bad faith and violations, there's a framework in place. As for violations of UN resolutions, you need to look at it in an issue-specific way. Resolutions on Israel may be violated by a set of countries whereas those on North Korea by another set of countries.
Why violations? (Technically, they are not violations because the resolutions are non- binding) Because, apart from UNSC resolutions, UN resolutions are not really enforceable in a meaningful way apart from a trust on good faith. And you know which countries are on the UNSC.
The system is not perfect, including the UNSC. It is often dysfunctional, however, it still provides a lot of regimes that make our life easier, from aviation to behavior during war time.
Indeed, some people argue the Plaza Accords were the underlying reason for Japan's stagnation/stagflation which lasted till Abenomics. Others, however, ascribe it to politically weak governments. Keep in mind that US forced not only Japan but also others such as West Germany and UK to sign similar accords, mainly involving currency appreciations vis a vis the USD.
I am not sure which coercive methods the US employed at the G5 meeting they had in 1085 since, in the early 80s, it was the US which objected France's suggestions to appreciate the USD whereas then US Treasury Secretary thought a strong USD proved strong US economy and prestige. Even before the US action, actually, Germany intervened by selling USD to help depreciate it value. I am not sure how happy Japan was with the decision, but, it was definitely not the US initially and solely.
I think Japan can make sovereign decisions but it, like any other country, does not make decisions in a vacuum. A game theory of cause-effect linkages play a role. They need to consider multiple factors. After all, it is a multi player game. US is strong, but not stupid to really go out and dictate. It will threaten on various ways, since every state seeks its own interests, but, if Japan thinks that the advantages of not giving heed to US demands outweighs that of disadvantages, it will simply not heed. It's all about cost-benefit calculation.
I believe this the case with US military presence. Or it was with the Plaza Accord. In fact, it benefitted immensely with the US willingness to share tech and know-how during its economic assent. Of course, the US too benefitted.