A: Yes, we plan to release new RubyMine builds and plugin updates synchronously. The plugin will remain free for those who have purchased a full IntelliJ IDEA license.
A: The Ruby plugin for IntelliJ IDEA is built from the same codebase as RubyMine. Some features may be temporary not available in the plugin because the latest available IntelliJ IDEA bases on an earlier IntelliJ platform compared to RubyMine.
A: RubyMine has simpler UI in a number of areas like project management, IDE settings etc.
The Ruby plugin, in its turn, has the full range of JRuby support features (cross-language navigation, completion and refactoring). RubyMine only supports JRuby as the runtime for running applications.
If you're not using the Java/Ruby integration, RubyMine is a better place to start.
A: No. The two products use separate licensing systems; you need to buy two licenses if you would like to use both products.