Catalyzes the methylation of C-15 in cobalt-precorrin-6B followed by the decarboxylation of C-12 to form cobalt-precorrin-7. Co-precorrin-6B + S-adenosyl-L-methionine = Co-precorrin-7 + CO2 + S-adenosyl-L-homocysteine Cofactor biosynthesis; adenosylcobalamin biosynthesis; cob(II)yrinate a,c-diamide from sirohydrochlorin (anaerobic route): step 8/10. Belongs to the methyltransferase superfamily. Bacterial-type CbiT family. Was originally thought to be a precorrin-8w decarboxylase.