It is important to note dataplane itself is just a platform for additional services. At the moment those services are Data Lifecycle Manager (DLM) and the upcoming Data Steward Studio (DSS) and Cloud Manager.
1) It doesn't contain those products (they are contained in HDP) but it leverages them through their APIs to make the data available to dataplane services
2) It allows a single pane of glass for data management across multiple clusters. DLM for disaster recovery and data movement between clusters. DSS for metadata management and auditing.
3) DLM is currently available, DSS is in tech preview. Currently they are only available to Hortonworks customers, I don't know if there is a roadmap to connect to other hadoop distributions.
4) There is no sandbox environment for Dataplane at this time.