Hi ,Kindly refer the link https://experienceleague.adobe.com/docs/experience-manager-cloud-service/content/operations/indexing.html?lang=enfor creating custom index in AEMAACS. ThanksHimanshu

Scene7 queue is shared so make sure either you process in small batches as per your business priority or reach out to adobe support team to fine tune or allocate more resources for your program for scene7 sync. ThanksHimanshu

Add you custom workflow as post processing workflow .Refer: https://experienceleague.adobe.com/docs/experience-manager-cloud-service/content/assets/manage/asset-microservices-configure-and-use.html?lang=en#supported-file-formats ThanksHimanshu

Supported format https://experienceleague.adobe.com/docs/experience-manager-cloud-service/content/assets/file-format-support.html?lang=en 

In that case you use CUG and assign those users who need access for these 2 resources .So whenever they try to access they need to authenticate . https://experienceleague.adobe.com/docs/experience-manager-65/administering/security/cug.html?lang=en 

Refer: https://experienceleague.adobe.com/docs/experience-manager-learn/getting-started-with-aem-headless/authentication/overview.html?lang=en 

You can add extra options in the dialog multifield for each tab and add the icons accordingly .

Is this replicated to publish instance?

Hi @Fernanda_M_Motta this is for assets only . For page properties kindly refer https://experienceleague.adobe.com/docs/experience-manager-65/developing/extending-aem/bulk-editing.html?lang=en  

For Assets you can select the folder and export the metadata in csv format . After that you can make the edits and upload the same  by clicking create button and select  metadata option .