I noticed the same behavior. It looks like what gets added to the new girl is related to the base stats, not including the girl's bonus. For Indira, the commitment stat is kinda weak in the coaching because I don't practice to get base stats level, I look at the end result.
It looks like there is some threshold for being able to coach again.
Oh, and they definitely select your best girl based on base stat. I have 2 girls now with better gear giving them better stats, but Indira is still used as my "best girl"