Intergration engine will always use rffective user and effective group defined during IRIS/Cache installation. Your windows CIFS share also mounted with same user/group and at windows side these user/group are allowed write permissions if you are trying to write at the share location.

Thanks for the update. 

One last question, just shutdown Primay linux server (with 'shutdown' command while IRIS is up and running) and then see how long time Backup member takes to become Primary.

Does this is validated in a Mirror env?

When you restarts Primary Linux server then how long it took for a Backup memeber to become Primary?