She didn't defect when I played the chapter. I used her primarily to fend of Wyvern Riders. As for her father, I used Ike to initiate an attack, and then I used Reyson to rejuvenate Ike to land a finishing blow before he would heal. After the chapter, Ike apologized to Jill about her fathers defeat, and that was the end of it.
If she didn't defect in Chapter 20, I see no reason to think that she is just going to defect later on. In that case, I don't think I really need a backup save file in the case I might lose her.
I would like to thank you though.