kie-server provides "kie as a service" (configurable to support rules, processes and/or planning solutions).
kie-server provides a RESTful API to deploy KJARs and execution of content (rules, processes etc).
1. Yes, in 7.x you'd use kie-server for runtime managed from the workbench with RESTful API.
2. Yes, kie-server is mandatory if you want to run rules with a RESTful API "out of the box". No, if you want to write your own service you don't need kie-server (it is a service we provide "out of the box").
3. Hopefully a colleague will reply.. I don't know.
5. You describe use of Kie Workbench and kie-sever. We find it a good design.
6. Yes, you can use Maven and KIE's kie-maven-plugin to compile KJARs. Although a vanilla JAR with DRL (in src/main/resources), Java (in src/main/java) and kmodule.xml (in src/main/resources/META-INF) will also work.