According to Osho, kirtan can be purely inward—but only when mind and body have naturally separated, like the kernel from a dry coconut. For most people they’re still one, so begin with the body: sing, clap, move, let the body immerse so the mind can dive. Start where you are; later, inner kirtan will arise effortlessly without bodily involvement.
Yes—but unless your mind and body are already separate, use your body in kirtan so your mind can follow inside.