I am not the subject matter expert on this, but maybe there are alumni or MIDN, who experienced the course validation process recently.
First, each department will handle validations the way they choose and this might differ from other departments.
My assumption is that if there is a test available during plebe summer, you have to take it (regardless of how you did on AP/IB/etc.). If you pass, then you have the option to validate. If you do not pass and you have the AP scores, you could try and make an argument, but my guess is you will not get validation credit.
If there are no plebe summer tests, then your AP score might come into play. When I was at USNA, some departments actually gave MIDN previous final exams and if they passed, they validated. Other departments have other requirements IAW the validation policy.
Other than for a few courses (American Government comes to mind), I would not expect to just hand someone a piece of paper saying you scored well on an AP test or did outstanding in an equivalent college course without some type of determination by USNA (review the course syllabus, might ask you questions to determine if you actually understand the course content, take a previous final, etc.).
If you did not do well on an AP exam, but pass a validation exam or another validation protocol, I am sure that isn't held against you.