Add a custom property in default rail whose value will be checked and read-only by default and it query parameter would be assets which do not have expiry property.thanks

This does not seems to be related to AEM Assets as there is no such panel OOTB in AEM.Thanks

This is because that in AEM 6.3 DAM Update Asset workflow has been made transient. review more details here https://docs.adobe.com/docs/en/aem/6-3/develop/extending/workflows/best-practices.html#Transient Workflowsmake workflow untransient and trythanks

It depends on each organization policy. For example BFSI would never want to delete anything. You can review below for more understanding on AEM maintenance tasksAem maintenance Operations Dashboard Revision Cleanup Thanks,Gaurav

There is no such workflow OOTB. Could have been copied from DAM Update asset. You can check each workflow step and compare.Thanks,Gaurav

Should not be problem but you need to adjust references of path across wherever its used.Thanks

AEm supports PDF via some library and that library provides set of functionality. To edit PDF you need application which can understand format like to edit Video you need to have some app which understands video editing.Thanks

You can explore AEM desktop app Adobe Experience Manager Desktop App  via which yo ucan open PDF in their native applications on desktop, edit and save and changes would be updated in AEM.Thanks

You can not edit PDF in browser or some web app unless it has PDF code. PDF can only be edited in Adobe Acrobat (not Reader) or similar softwares.Thanks

Review RenditionMaker ("The Adobe AEM Quickstart and Web Application.")  and reviewcreateThumbnailTemplateThere is parameter which you need to pass.Also review adobe - Managing image renditions on CQ5 / AEM DAM upload for responsive pages - Stack Overflow Thanks