Hi Russ, very ambitious.
Though I am not a fan of CMU's, I do like the idea of an arched masonry root cellar.
I would recommend however that you study the art of arches before attempting any further building. My go to resource is Art of the Stone Mason by Ian Cramb, he has extensive treatment of arch building in a very hands-on way that only the truly knowledgeable can put forth and <$10.
I would not asphaltium anything! Wick the moisture out with hygroscopic coatings like lime plaster, but anything that blocks water vapor will be problematic.
Russ - are you going to earth berm, then plastic sheet, then top with more soil?
That is the way most wofati type structures seem to be done, so that dry soil is next to the structure. The top 50cm of soil over the plastic acts to protect the membrane from damage and punctures. You basically want as much soil as possible inside your membrane umbrella.
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