I think its better that Alba become the more attacking wing back bcos it simply favors Messi's left foot. Messi has scored more from the overlapping runs on the left (Neymar, Alba). However, in terms of linking up, 1-2s, he's better with Alves.
However, if Dembele were to play RW, Alba will have to take up the attacking role that Neymar left. Suarez could help out Alba but he doesnt have the speed so Alba will throw himself forward more. Defensively the left side will be compromised. Thats what Im worried abt.