I think you did a good job with the explanation, Darren. Jane Bison should definitely be listed as the mother since she is the birth mother. Explaining the foster mother situation and adding Alexander as an alternative surname will help anyone else who works on the profile in the future to understand why the Alexander surname was used rather than making the hasty (and incorrect) assumption that the mother is incorrect.
I suggest that you change the Current Last Name to Plummer so her married name is also listed and searchable.
Keep up the good work!