The key difference, is that the US is many jurisdictions (Federal + 50 states + a lot of others, from counties to cities to territories to MANY others), and the variance amongst those is high.
The key thing well regulated places like Sweden get right, is that in consumer contracts you have minimum bars that you must meet regardless of what you can get the consumer to agree to. So, for instance, return policies, for goods bought online have minimum standards they must meet.
In the US, these things have huge variability. There are well regulated states, and well, the others.
Yes, but Swedish contract law actually is like this. A contract is a specific agreement, it can never be "Oh well, you can add provisions as you like if you send them to me" or "I will pay whatever".
If you have an agreement that says one party can announce changes, you don't have a contract, because those changes were not agreed to.
To me the insane part is that contracts don't have to be registered with the courts (or some qualified third party) ahead of time.
Like each party could show up with their own piece of paper (or not be able to provide it). Which is largely the issue here in that one party is showing up with a 2021 document and the other a 2023 document.
Yes, of course.
We don't have any rules about contracts needing to be written down or registered or anything of that sort. Even verbal agreement are valid, and you are entering into simple contracts even when you buy something in a store.