Millbrex Church appears to have been built in 1833. From somewhere deep in my memory banks I seem to recall it was part of the Parish of Fyvie but got its own church due to the difficulties of getting to the Parish Church during winter.
http://scotland_topograph.enacademic.com/1630/Millbrex

Prior to this, I would image all burials would be in the main church graveyard.

ANESFHS have Indexed Fyvie & Millbrex graveyards. The Index is on-line at
http://www.anesfhs.org.uk/

Brian