Review org
Users can review changes in an org that contains the latest version of the project source with new changes also applied, as part of the Review step and prior to promotion. Review org is likely ephemeral and is created and used specifically for the purpose of review of a particular work item.
Yes ! This echoes what Wade Wegner tried to do with the initial Heroku Pipelines implementation.
Comparing components in the branch against the target org gives great flexibility to developers to work independently and not worry about overwriting the target org. Some of the DevOps tools in the market (flosum) provide those and pretty asset for the overall process.