MLB Rule: Courtesy Runners & Player Re-entry After Substitution

Question
According to rule: any starter or sub may re-enter the game once. My question is; "Can a starter (wearing #24 and batting 2nd) on the bench replaced by #6 come back in as a courtesy runner for (catcher batting #8)?" And if yes, "can she later come back in to replace #6?"

Answer
Hello Sir,

Thank you for your question.  I hope you don't mind that I changed your question from private to public.  We are allowed to do that if the question contains no personal information.  

No, starter #24, who was replaced by substitute #6, cannot re-enter as a courtesy runner.  Only bench players who haven't had any other role in the game (other than as courtesy runner!) can courtesy run.  Of course, #24 can re-enter for #6, or for #6's substitute, etc.

Scott Kelly