Scott,
I do not think you can configure the quality trend graph. You would need to create a custom report off the IS Repository tables. Same with the error counts. Its a good idea though so maybe you should raise to the Idea space (https://ideas.sap.com).
I raised the idea of filtering for specific areas (business units, locations, work groups, etc) with SAP as there are often different people responsible for resolving data. I believe there is an idea for this to improve the DQ reporting. The only way I could think of resolving is a custom DQ report and use BOE via report bursting to email users/groups with their items to fix. Hopefully this becomes standard in IS to avoid all this custom reporting.
No you cannot remove a score - again custom reporting.
You can download all failed data for a rule via Scorecard, rule tab for a single rule, or for a specific rule/binding or table/view via the Rule Results tab on the Workspace. Not sure there is a way to download all failed data without custom reporting on the failed rule repository.
regards
Adrian