The general opinion among the scholars I've spoken to about this is that they have a problem with "maes-" being of anything other than Norse origin, due to the overwhelming influence of Old Norse on other place names in Orkney. Personally, I don't think the "it must be Norse because everything else is" argument holds much water without further argument or evidence - a Norse origin for the "maes-" element would help, for example. The date when "Maeshowe" came to replace "Orkahaugr" would have considerable relevance, I feel.

Anyway, would you like me to email you the correspondence I've had with learned Doctors of Celtic history about this?