you can prevent some authors from accessing the rule editor. you cannot prevent people from accessing the rule editor and not making changes https://experienceleague.adobe.com/docs/experience-manager-learn/forms/adaptive-forms/restricting-rule-editor-aem-forms-technical-video-use.html?lang=en
Exactly, you can write custom code to store the generated pdf in s3 to start I would suggest storing a sample pdf in s3 using Java codeif that is working, convert that code into custom workflow component and use it in your process
for the first question about GDS pointing to S3, we have not tested that scenario so it's uncharted territory. You can store the generated PDF per your requirements and return that URL to the calling application.
Here is the link to the article which explains how to POST binary content using swagger file. In this tutorial download the article assets and upload them into your aem server. In that assets package is the swagger file which shows you how to send binary data