In September I plan.hope to travel first to Copenhagen for 4 days and then fly to Stockholm. Have read Sweden government info several times. Appears to say entry to Sweden from Denmark even by "foreign nationals" (does this include US citizens?) is without restriction. So tourists from US allowed? Without negative Covid test? I am fully vaccinated with US CDC card for proof. Thank you especially to anyone who has recent experience.
That is right, the border is open from the other Nordic countries, regardless of citizenship, from: https://polisen.se/en/the-swedish-police/the-coronavirus-and-the-swedish-police/travel-to-and-from-sweden
"From 31 May 2021 regular entry requirements are enforced for foreigners entering Sweden directly from Denmark, Finland, Iceland or Norway, meaning it is no longer necessary to be covered by an exemption or present a negative covid test. This is regardless of the foreigner's citizenship. "
That should be possible. But between Copenhagen and Stockholm the train is usually the better option.