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.

you can prevent authors from accessing rule editor using groups

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

We have a sample swagger file for attachment, let me send you the link 

use acrobat send to send the file and attach the download link here 

you can force the users to enter uppercase using the following regex [A-Z]+ in the field property 

currently it does not give any errorplease log a support ticket 

Theme editor needs AEM forms add on

I don’t think so BTW- what is the use case for that scenario?thanks 

Please contact your Account Manager or your Customer Success Manager