How often does an elbow disdlocation reduce itself?

Somewhat often. Elbow dislocations can spontaneously reduce themselves. These can be total dislocations or partial (subluxation). Evidence of the dislocation can be noted on physical examination, x-rays and mri. However, some elbow dislocations are locked in the dislocated position and require manipulation by a health care professional. Most are treated by early protected motion exercise. Sometimes surgery.
Radialhead yes. Radial head dislocations frequently can radiioulnar dislocation almost never by itself there are self reduction techniques however.