I was able to solve this myself. The script that works is:
var nRows = xfa.resolveNodes("Table1[*].Row1[*]");
for (var i=0; i<nRows.length; i++)
{
nRows.item(i).Subform13.AddQuestionButton.presence = "invisible";
nRows.item(i).Subform13.DeleteLastQuestionButton.presence = "invisible";
}