Yes, you can travel freely between those locations up until you enter the Hall of Echoes (which is the end section of D2:ED). BTW, the only thing in Rivertown Gorge is Kali's flying fortress.
In DKS (or D2:ED with FoV installed) you return to Aleroth after the end of the Hall of Echoes, though the city will be opened up and changes made to the areas accessible earlier in the game. No other previous areas are available in FoV (you can use the dragon stone to return to the battle tower, but the teleporter shrine there will be disabled, so the only exit is back to Aleroth).
