If I change a Helper layout, will it change where it's used?
No. Helpers are coupled with the page once placed. Changes to the Helper layout will not affect existing instances.
Acquia Cohesion renames to Site Studio from version 6.2. Tell me more.
You can use Elements to create content but we don't recommend doing so for multiple reasons.
Yes, this is possible by creating a custom element.
No site specific data is stored on the API.