They were not authorized to use the Circle of the Moon in the book (it is not part of what WotC has licensed), so I would say it would be possible to have them in the Scarred Lands. I haven't read the Incarnate class not am I really familiar with the Circle of the Moon so I would not comment any further on their interaction, but my bet is that the features of the Circle of the Moon don't apply to the Wild Incarnation (otherwise it would have been called Wild Shape instead).