May 2018
It turns out someone else has opened a similar bug report at this link: https://answers.ea.com/t5/Bug-Reports/Sim-Will-Not-Move-Out/m-p/6806380#M4114
I have added a "Me too" to that thread.
Thank you!
May 2018
Merged!
June 2018
I'm happy to report that with the recent update, I was able to move out my oldest Sim. Success!
I still have the issue with the "missing baby" issue, but at least the "unretireable Sim" issue is closed for me.
Thanks again!