> 1. Unless API explicitly guarantees that property, relying on that is bad idea. I wouldn't.

    With a sufficient number of users of an API,
    it does not matter what you promise in the contract:
    all observable behaviors of your system
    will be depended on by somebody.

Sure, but that's not really the point is it? If you get a UUID you can store it as a UUID. If the UUID happens to come around as a v7 you get some better behavior in your database, and if it does not, then it does not but there is nothing you can do about.