I have another thought that builds on my previous comment (#136 in this thread) and seems off-topic at first but eventually may bring us back to Mirri Maz Duur. I think GRRM wants us to compare Marwyn and Sam Tarly. Not just because they are both in the Maester business, although that is significant. We can also see that Sam is a living link between Maester Aemon, who explains part of the "dragon revival" prophecy in Sam's presence, and Marwyn who seems interested in getting to Dany's drago