which account am I suppose to add on "Contnent1"The account will be Content1. @Content1_R is the name of a group in LDAP, which grants its members R permission to the Content1 account.
And also what should be the "SecurityGroup" for "Content1" since "SecurityGroup" is mandatory for a content item check in.You may have to create a generic group where all users have RWD permissions - resulting permission are intersection of those from SG and accounts.
Also once the account is added I cannot change the permission for a particular user, so the option left with me will be to assign him to different account which has desired permission for given content (that too if I can add multiple accounts).Account is a setting on a content item, and it is expected to be changed only exceptionally. What you can change, though, is membership of users in your created groups - thus, granting/revoking permissions of users to particular accounts. This can be as dynamic as you need.