Ferdinand having Louis's son in his custody was not equal to Ferdinand being able to succesfully exert his authority over whole Hungary. Frederick III had Vladislaus the Posthumous in his custody, but he wasn't able to be his regent in Hungary and John Hunyadi ruled instead of him. I think that it would be similar situation.If Ferdinand has the child then that could be an issue for Zápolya. From the explanation of how Hungarian succession laws work or not work - the 'if it's too difficult sweep it under the carpet and elected who we want' approach worked because everyone ignored the reality of the situation. What if this time Ferdinand's allies in the Hungarian Nobility challenge the election claiming that Zápolya could not be King as Louis had an heir... Is that something that would be possible or is it simply the case that the Nobility have spoken and so it be done.