1. You are right that typically you need two registrational trials for approval and yes you can have many trials that show it doesn't work.
2. You are incorrect that you don't need to show it works against a disease. Sometimes you can show that you are impacting a biomarker (which I assume is what you mean by "some number"), but there must be evidence that changing the biomarker impacts the disease. Without that evidence you will not get approval.
3. You are incorrect you only need to show its effective for up to six weeks. If that were true, why would drug companies be running multi-year trials for cholesterol drugs? You typically need to show its effective for however long the drug is taken, but a follow-up period is also required.