Can yeast ferment maltose directly?
The statement in your first paragraph is correct: for yeast to ferment a carbohydrate it must be fed into the glycolytic pathway which, in the case of oligomers of glucose (e.g. mannose) simply means breaking the oligomer into its constituent glucose monomers using a single enzyme. This is similar to how yeast processes sucrose: the disaccharide is hydrolysed by invertase to release glucose (fed directly into glycolysis) and fructose (fed into glycolysis via fructokinase).

A separate mannose pathway would require a completely new set of enzymes that somehow processed mannose in a stepwise fashion without breaking the glycosidic bond linking the two monomers - I'm sure that you can see that this is a much less likely option in terms of the evolution of yeast metabolism.