Catalysis of the reaction: acyl-CoA + 7n H+ + n methylmalonyl-CoA + 2n NADPH = n CO2 + n CoA + n H2O + multi-methyl-branched acyl-CoA + 2n NADP+.
acyl-CoA:methylmalonyl-CoA C-acyltransferase (decarboxylating, oxoacyl- and enoyl-reducing)