Maybe this article will be helpful: How to integrate 3rd party consent tool with Piwik PRO anonymous tracking?
According to the docs, it notifies the system about the fact, that user has already seen the form and did not submit it. It explains the fact of showing a reminder. You can hide the reminder by using setComplianceSettings.
Yes. We verify it currently by using sharing secret. Unfortunately, it’s available only in the paid version.