According to Osho, a Tirthankara who chooses to return must preserve, before death, a single powerful vasana—the intent to teach—in the fourth body. This retained link keeps the bridge to rebirth intact, enabling descent into a physical body despite entry into the fifth. Without that peg of intention, return is impossible.
Yes—if, out of compassion, they keep one last strong wish to teach, they can come back in a real human body.