Hi there,
On Janie’s 1949 death cert it gives the then current details of her 3 brothers and 1 sister. (their names and addresses with the note "All of Johannesburg"). Have you followed up on those clues? Perhaps they or their descendants may know of your father’s life in South Africa, or of any previous marriage/s for him.
ADD,
It also mentions that Janie's Nationality was Poland, "Union National by naturalization of husband", so perhaps you could follow up re Isaack's naturalization papers.
ADD
One of the brothers shared the same address as Janie and Isaack.
Cheers, JM