My ex's niece has Williams syndrome, I have never heard of any cardiac problems meantioned, she is high functioning but you can certainly tell she has a mental disability, as well as those elfin features. She has a lovely, friendly personality and a great memeory she is around 35 now and for a while she moved into a shared house situation but is very much a home body and likes routine and does'nt do well with change so she did'nt last long, but certainly could have handled any tasks required as far as home duties went.
Best of luck, I am sure it will be a relief to know for sure.