How big is the app? How much usage will it get in staging?
If you spin it up on a t2.micro, the extra cost will be negligible, especially if you use auto-shutdown or Terraform to only have the box running when you need it.
It's critical to have separate environments for staging and production.
Finally, consider dockerizing your app if you can. Much more ...
You can approach this case with multiple ways.
For example you can set an auto-shutdown for 20:00 pm and auto-start for 8:00 am.
Use Terraform to create your environment and, after deployment/testings, you can easily destroy it, saving your costs.
And many other ways.
I commented on paths that you can follow, so you should choose one that brings you ...
Not technically, I understand your need to create and destroy EC2 instances, whether it's done via automation or not. About having permissions at the security group level, this can often be limited to viewing only the resource, as it is a networking firewall resource, usually who manages these objects are SDN or NaaS
Restoring a snapshot simply creates a new RDS instance.
So if you restore the prod DB snapshot to staging then you will be creating a new RDS instance with the restored data in it.
If both of the RDS instances are in the same aws account then
You will either have to delete/rename the staging DB and then restore the prod DB
If both of the RDS instances are ...