If a Swift class is made available to Objective-C code (using the @objc annotation, or subclassing an Objective-C class), that class will use message sends for calling methods. Using an Objective-C class, or a @objc Swift class, from Swift will also use message sends for method calls.

Yes. Run some Swift code in Xcode with the debugger and step through its execution; you can see that Objective-C objects proliferate through Swift. Cocoa APIs are especially Objective-C compatible, as to be expected.