You are right, he was parentally abducted from the Minister's care, at that point in time.
Which resulted in a delay of 6 or so weeks before William was placed into the hands of his waiting carers.
I believe that the FF would be aware that the BM was opposed to the permanent placement.
The possibility of another 'intervention' by the BF would likely not be far from the FF's minds. imo
"William Tyrrell was always intended to stay in foster care for the rest of his childhood, away from his 'career criminal' father.
The young missing boy's mother ..... was also believed to have known of the intention for the arrangement."
William Tyrrell's foster placement was to be permanent