Henesua is correct, inventories of placeables and the inventories of merchants waypoints get loaded when a PC enters the map they are found on. This can add to an areas loading times. If this made stores load faster when you accessed them, any extra load time would likely be moot. It doesn't though. When you open a store, all the inventory info is resent anyhow. So in the end, having stores on a single map that is inaccessible to players works just fine and can help optimize a module when combined with other lag reducing practices.
To enable merchant waypoints to work anyplace in your module, just change
object oStore = GetNearestObjectByTag("Store_"+sStore);
to
object oStore = GetObjectByTag("Store_"+sStore);
in your open store scripts.