I'm not Dutch, but I have allowed myself to be told that in the Dutch university system nearly every applicant gets accepted, and those with insufficient course credits get kicked out at the end of first year. Is there even a hint of truth in that?

It's mostly true. Most universities require that incoming bachelor's degree students to have finished either a matching pre-university secondary education program or pass a test (only for people of 21+ years old) that checks whether a student has the required Dutch/English/math skills. After this, universities are allowed to boot students that fail to pass 50, 80 or 100% of the courses in the first or second year (actual percentages differ per university).

On the other hand, master's degree programs tend to require a bachelor's degree that has sufficient overlap in terms of courses. So to start a Computer Science master's degree, you'd probably need some bachelor's degree in engineering, math or physics. If you for instance got a bachelor's degree in Psychology, you'd probably need to attend several courses in order to be allowed to join.

Yes, with the caveat that only 20% of Dutch high school students attend secondary schools that have "go to university" as their desired outcome. Virtually everyone who successfully completes that secondary programme gets into a university (the variation in quality between them isn't nearly as great as it is in the Anglo countries) but it obviously isn't fair to compare that 20% with everyone who graduates an American high school.

Changing majors in the US is very common. I don't have any numbers but I'd guess 30-50% of all US college students have changed their major at least once.