This is a bit drastic but here goes....
Assuming that the hatch (upper and lower) opens completly and will stay up (or can be braced to be open), remove the rear seat out the hatch. This will give you a bit more manuvering room. Next, unscrew the upper trim molding on the door. You now should be able to access the fitting on the inside door panel and CAREFULLY pry out each of the fasteners. With the inside of the door area now exposed, check the latch assembly and lube it up good...you will find you can hit the latch with lubricant from all kinds of angles. Also you can check to see if any part of the remote latch assembly (the push-button opener and other areas) is badly coroded or otherwise will not move. A stuck part of the assembly can prevent latch movement of the type you describe.
The passenger side rear seat door is NOT welded shut unless an owner did it to use the car more as a truck. Since the driver side rear door is welded shut at the factory also welding the passenger side shut would defeat the entire purpose of having a rear seat for people. Bending with a prybar runs the risk of bending the door frame on the shell, the door itself as well as the latch mechanism.
