Today I spoke with both a family law lawyer and an immigration lawyer on an informal, hey-what-do-you-think basis (ie not asking for a researched legal opinion, but what they thought. I even told them to assume Brad was factually innocent (though also I thought he was guilty). Neither had heard of this case before, but I forwarded them the link to the Dateline episode as they were curious and somewhat shocked I knew the perpetrator).
-The immigration lawyer commented that the girls would be automatic US citizens, as well as eligible for Canadian citizenship as the children of Canadian parents, but an application would have to be made. She noted that certain benefits in Canada require legal status in Canada, and so she couldn't imagine that these girls did not already have formal Canadian citizenship since they have been living here for some time. Whether they had it before Nancy died or after as a result of the Rentz's taking then-temporary custody is unknown. She also commented that Brad would be removed from the US upon release and so will no longer have access to the US for anything, including court proceedings.
-The family law lawyer commented that because the girls will be residing in Canada and by the time Brad is released, will have been for over a decade, there is little doubt that only the Canadian courts will take jurisdiction in any case involving custody of the girls. She pointed out that once released she thought Brad would be removed from the US and so there would be no reason for the US courts to take jurisdiction--- the custodial parents live in Canada, the girls live in Canada, and Brad can not enter the US. She said Brad would not have standing to bring any court application assuming a completed adoption, regardless of what jurisdiction that adoption happened in. Even then, by age 14 and 16 there is no way Brad would be eligible for custody, as he will have been out of their lives for a long time. Even access would not be in their best interests as, whatever the truth is, neither child has any relationship with Brad and the controversial disruption to their lives would only cause pain and frustration. Whether to allow contact with Brad would be a decision only the girls can make, and once again, only once they are adults. The actual conviction for killing their mom is important, but not necessary to prove to deny access.... even if Brad could get over the issue of not having standing, he will have serious problems in convincing the court the disruption would be in their best interests even being the biological father.
-I mentioned the duress argument to the family law lawyer, she thought it would be something she would ever recommend to even try.
Anyway, while these aren't legal opinions, they provide some insight.