Wondering too how this is going - just read through it this evening. Sorry for your sister, sorry for this basenji.
Wiser heads than mine have already given you insightful answers. All I can say is that we dealt with similar issues with our rescued boy. He suffers from fear aggression and I have lingering pain in my knee to this day from a bite shortly after we adopted him. (Not as bad as your sister's - I can only imagine how dealing with that situation in a contained space at 60 mph must have escalated things.) That aside, our guy is an absolute sweetheart, much more affectionate than our well-behaved girl. It took many, many slow months of gaining trust and training to get there, but if it works out it's worth it.
As others have said, whichever way your sister decides to go - if she's given the choice by AC - is understandable. If she ends up with this boy in her home she may be in for a long haul, but the payoff could be huge. Warm wishes to her - and to you for caring - however things end up.