hi @suresh1
last November, QuickSight released new set of APIs which allows to have all the visual information exported with JSON format. you can use those APIs to see how multiple datasets are handles in each visuals as well as sheet information. with these APIs, you can export dashboard and import into another account as well. the details of APIs are described in blog : New Amazon QuickSight API Capabilities to Accelerate Your BI Transformation | AWS News Blog
as for database you use in account A, it depends on what database you are using. if you are using Redshift or RDS, cross account access is supported and the below is blog.