2) Any item that is not legal there will be just void in court. You cannot be sued about an invalid legal policy, but only after breaking the law. The policies do not subsume law.
About the only thing you need to publish is which data is collected, how it is processed (and by whom if outsourced), for how long (if applicable) and how to remove it.
3) Uh, as usual complying to the law for PII handling?
4) Yes, if they are GDPR compliant. Make sure to put them in you privacy policy.
5) Yes, if the source is GDPR compliant.