Hey if anyone has an answer to this I could really use it because this appears to be game-breaking;

I’m in the Old Dungeon right now, I just freed the Cyclops and finished the Isle of Hajar without pulling all of the levers in the same area with the cells that you find the Cyclops. For whatever reason, no matter what I do, I can’t open the cell all the way at the end and top (the northeast most over with the zombies). I’ve moved the rock onto the pressure plate in the hallway, hell I even grabbed another rock and put it on top of the first pressure plate in the hallway so that they’d both be down - nothing. Everything I’ve tried hasn’t worked, the main character just slashes at the door without opening it.

I seriously need to get this resolved because otherwise I have to start the entire game over which I really don’t want to do. Any help is greatly appreciated.

Using only a save file is a bad habit.
However, even if I am not sure I correctly understood which door you are talking of, I don't think you really encountered a game bug: I suspect you simply skipped one of the steps necessary to open it.
I suggest you to watch this video and maybe the next one, if necessary.

[SPOILER]
Basically you should see six cells and six pressure plates: in each cell there is a lichen covered wall which conceals a lever. Each pressure plate activates the nearest wall, so you have to cover a plate, go for the nearest lichen covered wall, hit it until it reveals a lever and finally pull it. You have to repeat the procedure for each lever, until all six are pulled, in order to unlock the door.
[/SPOILER]

I watched the video you sent - the room I can't open is the one he enters at around 12:50. I've done everything right but no matter what I do it just won't open.

Sorry about that: I'm afraid I can't help you then.