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.
3.) No, unfortunately it isn't that easy. Some people - lawyers even - argue that merely someone contacting you via email or handing you a business card doesn't necessarily constitute legitimate interest on your part to process their contact data for the purpose of contacting them in the future. I disagree with that opinion but that people are even arguing about this shows that this isn't just business as usual.
5.) You could argue that this has the potential for breaking how the web has worked until now. If you now have to check for legal compliance first each time before merely linking to an external resource (because that might reveal the user's IP address) that simply doesn't scale. Linking to and drawing upon external resources arguably is what makes the web the web.